Worksheet Grade 7 Health & Consumer Science

Harwick's Lead Tap Test: Sampling and Probability

A grade 7 worksheet using a fictional town's lead tap results to find the probability a random tap exceeds the action level and decide what to do.

Grade: Grade 7
Subject: Health & Consumer Science
Material: Worksheet
Time: 35 min
Prep: No prep
Materials: None

Standards

7.SP.C.5 Common Core Math

Understand that the probability of a chance event is a number between 0 and 1 that expresses the likelihood of the event occurring, with larger numbers indicating greater likelihood.

How this material aligns: Items 2, 5, and 8 have students write and use a probability between 0 and 1 that a random tap exceeds the lead action level, exactly the probability meaning this standard names.

All materials for 7.SP.C.5 →

7.RP.A.3 Common Core Math

Use proportional relationships to solve multistep ratio and percent problems. Examples: simple interest, tax, markups and markdowns, gratuities and commissions, fees, percent increase and decrease, percent error.

How this material aligns: Items 3, 6, and 7 convert those probabilities to percents and find the percentage-point change when the action level drops, the multistep percent work this standard requires.

All materials for 7.RP.A.3 →

Download Word file

This material is in the public domain (CC0) and may be copied, edited, and shared freely, without attribution. Updated 2026-07-17.

Objectives

Students will write the probability that a random tap exceeds the lead action level.
Students will convert probabilities to percents and find a percentage-point change.
Students will list household actions to take when a tap tests above the action level.

Vocabulary

Action level: A trigger number in lead and copper testing. If too many homes test above this level, the utility must take steps like improving treatment and telling the public.
Lead (Pb): A soft gray metal that is poisonous, especially for children. It can get into drinking water from old pipes and plumbing, so utilities work hard to control it.

Also: probability, sample

Teacher overview

Water utilities test taps for lead and report the results, but a single result is hard to read without knowing what "above the action level" really means. This worksheet gives students a fictional set of Harwick tap results and asks them to turn counts into probabilities and percents, then decide what a household should do when its own tap tests high. The scenario also shows how the coming drop in the action level changes the share of homes counted as too high.

Harwick's Lead Tap Test: Sampling and Probability — Grade 7 Health & Consumer Science

Name: ____________________________ Date: ______________

Student Worksheet

The town of Harwick tested tap water at 250 fictional homes. Lead was above the 15 ppb action level at 30 of them. The action level is set to drop to 10 ppb in 2027. At the lower 10 ppb level, 45 of those same 250 homes would count as too high. The action level is a trigger for the utility to take action, not a safe level; EPA's health goal for lead in drinking water is zero.

The lead action level today is 15 parts per billion. Write it with its unit.
Of the 250 homes, 30 were above 15 ppb. What is the probability that a randomly chosen tested home is above the action level? Write it as a decimal between 0 and 1.
Write that probability as a percent.
Is a tested home more likely to be above the action level or at or below it? Use your probability to explain.
In 2027 the action level drops to 10 ppb, and 45 of the 250 homes would be too high. What is the probability now, as a decimal?
Write the 2027 probability as a percent.
By how many percentage points did the share of too-high homes rise when the level dropped from 15 ppb to 10 ppb?
Suppose Harwick has 500 homes in all. Using the 2027 probability of 0.18, about how many homes would you expect to be above 10 ppb?
A household's tap tests above the action level. List two actions it can take right away to lower the lead in the water it drinks.
Explain in one or two sentences why testing only one home cannot tell you the lead risk for the whole town.

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

Answer Key

Answer: 15 ppb. The lead action level today is 15 parts per billion.
Answer: 0.12. 30 homes divided by 250 homes is 0.12.
Answer: 12 percent. 0.12 written as a percent is 12 percent.
Answer: At or below is more likely. The probability of being above is only 0.12, which is well under 0.5, so most tested homes are at or below the level.
Answer: 0.18. 45 homes divided by 250 homes is 0.18.
Answer: 18 percent. 0.18 written as a percent is 18 percent.
Answer: 6 percentage points. 18 percent minus 12 percent is 6 percentage points.
Answer: 90 homes. 0.18 times 500 homes is 90 homes.
Answer: Run the cold tap for a bit before drinking, and use a filter certified to remove lead; using cold water for cooking also helps.
Answer: One home may be higher or lower than the rest, so a single sample is not representative; the town needs many taps tested to judge the real risk.

Differentiation

Support

Provide the first probability worked through and a reminder that a probability is a fraction of the total written as a decimal. Allow calculators, and let students shade a 10-by-10 grid to picture 12 out of 100.

Extension

Ask students to explain why the number of too-high homes rises when the action level drops, even though no home's actual lead level changed. Have them connect this to why the rule is becoming stricter.

ELL supports

Pre-teach "action level," "probability," and "sample" with a simple picture of taps, some marked above the action level and some marked below it. Provide a sentence frame for item 10: "One home cannot represent the town because ___."