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Lesson plan Grade 11 Health & Consumer Science

Auditing the Escalante Family's Tiered Water Bill in Otterbrook

A grade 11 lesson where students audit a fictional family's tiered water bill, find cost per thousand gallons, and test it against a widely used affordability threshold.

Objectives

  1. Students will calculate a tiered water bill from a rate schedule and a household's metered use.
  2. Students will write and solve an inequality that keeps a water bill within a stated budget.
  3. Students will evaluate a household's combined bill against a widely used affordability threshold.

Vocabulary

Key words: tiered rate, marginal price, fixed service charge, affordability threshold

Teacher overview

Most households pay for water under a tiered rate structure, where each additional block of water costs more per thousand gallons than the last, yet few residents ever check how their own use maps onto those tiers. In this lesson students audit the bill of the fictional Escalante family, served by the equally fictional Otterbrook Water Works, and turn a monthly statement into a set of calculations they could repeat on any real bill. Combined US water and sewer bills rose about 24% from 2019 to 2024 and about 5.1% in 2025, so reading a bill closely is a practical consumer skill. A common misconception this lesson addresses is that the price per thousand gallons is a single flat number; under tiered rates the average price and the marginal price of the next thousand gallons are different, and confusing the two leads to wrong estimates.

Procedure

Hook (5 min): Project a blank monthly water statement and ask students to predict, in writing, whether a household that doubles its water use would see its bill exactly double. Collect a few predictions without confirming, then explain that higher tiers push the usage charge up faster than use while the fixed charge pulls the other way, so whether the bill more than doubles depends on the rates and the amount used.

Direct instruction (12 min): Using a fictional sample resident who used 5,500 gallons in Otterbrook (fixed charge 18 dollars; Tier 1 covers the first 4,000 gallons at 4 dollars per 1,000 gallons; Tier 2 covers 4,001 to 10,000 gallons at 7 dollars per 1,000 gallons), model the bill on the board: Tier 1 is 4,000 gallons at 4 dollars per thousand, or 16 dollars; Tier 2 is the remaining 1,500 gallons at 7 dollars per thousand, or 10.50 dollars; add the 18-dollar fixed charge for a water bill of 44.50 dollars. Emphasize that only the gallons inside each tier are charged at that tier's rate.

Guided practice (12 min): Distribute the handout and work items 1 through 3 together, splitting the Escalante family's larger use across all three tiers and adding the sewer charge. Have students state, out loud, which tier each block of gallons falls into before multiplying.

Independent practice (18 min): Students complete handout items 4 through 8 on their own, including the average-price calculation, the marginal-price comparison, the affordability-threshold check, and the budget inequality in item 7. Circulate and check that students divide by thousands of gallons, not by gallons, when finding cost per thousand.

Formative check and close (8 min): As an exit ticket, students answer one question on a card: "A neighbor says every 1,000 gallons costs the Escalante family the same amount. Using your item 4 and item 5 answers, explain in two sentences why that is not true." Collect and scan for students who correctly distinguish the average price (about 8.28 dollars per thousand) from the marginal Tier 3 price (11 dollars per thousand).

Auditing the Escalante Family's Tiered Water Bill in Otterbrook — Grade 11 Health & Consumer Science
Name: ____________________________ Date: ______________

Student Handout

Otterbrook Water Works bills the fictional Escalante family each month as follows:

  • Fixed service charge: 18 dollars
  • Tier 1, first 4,000 gallons: 4 dollars per 1,000 gallons
  • Tier 2, next 6,000 gallons (4,001 to 10,000): 7 dollars per 1,000 gallons
  • Tier 3, all use above 10,000 gallons: 11 dollars per 1,000 gallons

Last month the Escalante family used 12,500 gallons. Their combined household income is 3,600 dollars per month, and their monthly sewer charge is a flat 32 dollars.

  1. How many of the family's 12,500 gallons fall in Tier 1, in Tier 2, and in Tier 3?
  2. Find the water portion of the bill: the fixed charge plus the cost of each tier's gallons.
  3. Add the 32-dollar sewer charge. What is the total combined water and sewer bill for the month?
  4. Find the average cost per 1,000 gallons for the water portion only (water bill divided by thousands of gallons used). Round to the nearest cent.
  5. The marginal price is the cost of the next 1,000 gallons. If the family used exactly 1,000 more gallons (all in Tier 3), how much would the water bill rise?
  6. A widely used rule of thumb, built from two EPA benchmarks, treats a combined water and sewer bill above about 4.5% of household income as a high burden (EPA has no single official cutoff and has also used 3%). What percent of the family's 3,600-dollar monthly income is their combined bill from item 3? Round to the nearest tenth, and state whether it is above or below 4.5%.
  7. Assume the family stays above 10,000 gallons, so every extra 1,000 gallons is billed at the Tier 3 rate. Write an inequality using g for total gallons that keeps the water-only bill at or below 90 dollars, then solve for g.
  8. Explain why a tiered rate charges more per 1,000 gallons as a household uses more, and how that pricing encourages conservation.

From Water for Schools (waterforschools.org). This material is in the public domain (CC0) and may be copied, edited, and shared freely.

Answer Key
  1. Answer: 4,000 gallons in Tier 1; 6,000 gallons in Tier 2; 2,500 gallons in Tier 3. The first 4,000 fill Tier 1, the next 6,000 fill Tier 2 up to 10,000, and the remaining 2,500 fall in Tier 3.
  2. Answer: 103.50 dollars. Tier 1 is 4 times 4 dollars = 16 dollars; Tier 2 is 6 times 7 dollars = 42 dollars; Tier 3 is 2.5 times 11 dollars = 27.50 dollars; plus the 18-dollar fixed charge: 16 + 42 + 27.50 + 18 = 103.50 dollars.
  3. Answer: 135.50 dollars. Water portion 103.50 dollars plus the 32-dollar sewer charge equals 135.50 dollars.
  4. Answer: 8.28 dollars per 1,000 gallons. 103.50 dollars divided by 12.5 thousand gallons is 8.28 dollars per thousand, rounded to the nearest cent.
  5. Answer: 11 dollars. The next 1,000 gallons are all in Tier 3, priced at 11 dollars per 1,000 gallons, so the bill rises by 11 dollars.
  6. Answer: 3.8%; below the threshold. 135.50 divided by 3,600 is 0.0376, or about 3.8%, which is below the 4.5% high-burden threshold. It would be above the 3% measure EPA also used.
  7. Answer: g is at most about 11,272 gallons. The water bill above 10,000 gallons is 18 + 16 + 42 + 0.011(g − 10,000) = 76 + 0.011(g − 10,000). Setting 76 + 0.011(g − 10,000) ≤ 90 gives 0.011(g − 10,000) ≤ 14, so g − 10,000 ≤ 1,272.7, and g ≤ about 11,272 gallons.
  8. Answer: Accept any answer explaining that higher tiers price each additional block of water above the last, so heavy users pay a rising marginal price, which gives households a financial reason to cut back and rewards lower use with a lower average price.

Differentiation

Support

Provide the tier breakdown for item 1 already filled in, and give a worked template for item 2 that labels each multiplication (fixed, Tier 1, Tier 2, Tier 3) with a blank for the product. Allow a calculator throughout and accept unrounded answers on item 4.

Extension

Ask students to redesign the Escalante family's use so their combined bill lands just under the 4.5% affordability threshold if the household income dropped to 2,800 dollars per month, stating the maximum gallons they could use and justifying it with the tier math.

ELL supports

Pre-teach "tier," "fixed charge," "marginal," and "affordability" with a simple labeled diagram of a bill split into stacked blocks of increasing price. Provide sentence frames for items 6 and 8, such as "The bill is ___ percent of income, which is ___ the threshold because ___."

Extensions

Invite students to bring a real, name-removed water bill from home (or use a sample bill their local utility publishes) and repeat items 1 through 6 with its actual rate schedule, then compare their household's average cost per thousand gallons to the Escalante family's. A short class discussion can compare how different local rate structures reward or penalize the same level of use.