Water for Schools
Menu

Lesson plan Grade 7 Health & Consumer Science

Reading the Okafor Family's Dunmore Water Bill After a Rate Increase

A grade 7 consumer-math lesson where students read a fictional household water bill, compute a two-step percent rate increase, and judge whether it is affordable.

Objectives

  1. Students will read a household water bill and identify the fixed charge and the volume rate.
  2. Students will compute two successive percent increases and show why they do not simply add.
  3. Students will judge whether a monthly bill is affordable using a stated income benchmark.

Vocabulary

Fixed charge
The part of a water bill that stays the same every month no matter how much water a family uses. It helps pay for the pipes, meters, and workers the utility needs even when no water is flowing.

Also: volume rate, percent increase, affordability

Teacher overview

A monthly water bill is built from two numbers: a fixed charge that stays the same and a volume rate charged per thousand gallons. This lesson uses a fictional bill from the Okafor family in the town of Dunmore to show how those two parts set the total, and what happens when the utility raises its rate. When Dunmore raises its volume rate by 15 percent one year and 10 percent the next, students work through a common misconception: two increases of 15 percent and 10 percent do not total a 25 percent increase, because the second percent is applied to an amount that has already grown. The lesson closes by asking students to judge affordability against a real benchmark.

Procedure

Hook (5 min): Project the fictional Dunmore bill. Ask students to circle every dollar amount they see and to guess, without calculating, which one they could change by using less water. Take a few answers aloud. Confirm that the fixed charge stays the same no matter what, while the volume charge depends on how much water the household uses.

Direct instruction (12 min): Define the two parts. Dunmore charges an $18.00 fixed monthly service charge plus a combined water-and-sewer volume rate of $6.00 per 1,000 gallons. The Okafor household used 6,000 gallons last month. Model the base bill: 6 thousand-gallon units times $6.00 is $36.00, plus the $18.00 fixed charge, for a $54.00 total. Then introduce context: across the country, combined water and sewer bills rose about 24% from 2019 to 2024 and about 5.1% in 2025, so rate increases are a real household concern.

Guided practice (13 min): Walk the class through the two-step increase on the volume rate. Year one raises the $6.00 rate by 15 percent: $6.00 times 1.15 is $6.90. Year two raises the new rate by 10 percent: $6.90 times 1.10 is $7.59. Ask the check-for-understanding question: "Is $7.59 a 25 percent increase over $6.00?" Have students compute (7.59 minus 6.00, divided by 6.00) and confirm it is 26.5 percent, not 25 percent. This is the formative checkpoint; if more than a few students still expect 25 percent, pause and re-model the second step.

Independent practice (12 min): Students work questions 1 through 4 on the Student Handout: the base monthly bill, the new volume rate after both increases, the total percent increase, and the new monthly bill. For the new monthly bill, using 6,000 gallons of use, 6 times $7.59 is $45.54, plus $18.00, for $63.54. They then find the yearly total and compare it to the fictional household income of $16,000 per year to decide whether the bill is a high burden.

Closure / exit ticket (8 min): Each student answers question 5 on the Student Handout, the exit ticket: "The Okafor yearly water bill is about $762. Their income is $16,000. What percent of income is that, and is it above the 4.5 percent high-burden benchmark?" Collect and scan for the correct method before dismissal.

Reading the Okafor Family's Dunmore Water Bill After a Rate Increase — Grade 7 Health & Consumer Science
Name: ____________________________ Date: ______________

Student Handout

Use the Okafor family's fictional water bill from the town of Dunmore to answer each question, and show your work. Dunmore charges a fixed monthly service charge of $18.00 plus a volume rate of $6.00 for every 1,000 gallons of water used. Last month the Okafor household used 6,000 gallons.

  1. Find the Okafor family's total water bill for last month. Add the $18.00 fixed charge to the volume charge for 6,000 gallons at $6.00 per 1,000 gallons.
  2. Dunmore raises the volume rate by 15 percent in the first year and then by 10 percent in the second year. Starting from $6.00 per 1,000 gallons, find the new volume rate after both increases.
  3. Find the total percent increase in the volume rate, from the old $6.00 rate to the new rate you found in question 2. Is the total increase exactly 25 percent? Explain why or why not.
  4. Using the new volume rate and the same 6,000 gallons of use, find the Okafor family's new monthly water bill. Include the $18.00 fixed charge.
  5. Exit ticket: The Okafor family's yearly water bill is now about $762, and their household income is $16,000 per year. What percent of their income goes to the water bill, and is that percent above the 4.5 percent high-burden benchmark?

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: $54.00 per month. The base bill is 6 units times $6.00, which is $36.00, plus the $18.00 fixed charge.
  2. Answer: $7.59 per 1,000 gallons. First $6.00 times 1.15 is $6.90, then $6.90 times 1.10 is $7.59.
  3. Answer: 26.5 percent. From $6.00 to $7.59 is a $1.59 rise; $1.59 divided by $6.00 is 0.265, or 26.5 percent, which is larger than 25 percent because the 10 percent applied to the already-raised $6.90.
  4. Answer: $63.54 per month. After both increases, 6 times $7.59 is $45.54, plus the $18.00 fixed charge.
  5. Exit ticket answer: About 4.8 percent, which is above the benchmark. The yearly bill of about $762 divided by $16,000 is 0.0476, or roughly 4.8 percent, above the 4.5 percent high-burden line.

Differentiation

Support

Give students a partly worked bill with the base total already computed, so they only apply the two percent increases. Provide a labeled arrow strip reading "raise by 15 percent: times 1.15" and "raise by 10 percent: times 1.10" to reduce the load of remembering the multipliers.

Extension

Ask students to find the single percent increase that would take the old volume rate straight to the new one in one step, then explain why a utility might phase an increase over two years instead of raising the rate all at once. Have them justify their reasoning in two or three sentences.

ELL supports

Pre-teach "fixed charge," "volume rate," and "percent increase" with a simple annotated picture of a bill. Provide sentence frames for the affordability judgment: "The yearly bill is about ___ dollars. That is ___ percent of income, which is above or below the benchmark."