Math problem set Grade 6 Math

Harlow Reservoir: Storage, Days of Supply, and Conversions

A grade 6 math set where students convert a fictional reservoir's storage between acre-feet, cubic feet, and gallons, then find how many days of water the town has left.

Grade: Grade 6
Subject: Math
Material: Math problem set
Time: 45 min
Prep: No prep
Materials: Calculator

Standards

6.RP.A.3 Common Core Math

Use ratio and rate reasoning to solve real-world and mathematical problems, e.g., by reasoning about tables of equivalent ratios, tape diagrams, double number line diagrams, or equations, including finding a percent of a quantity and converting measurement units.

How this material aligns: Problems 1, 2, and 5 convert measurement units with ratio reasoning between acre-feet, cubic feet, and gallons, using the standard conversion constants stated in the problems.

All materials for 6.RP.A.3 →

6.NS.B.3 Common Core Math

Fluently add, subtract, multiply, and divide multi-digit decimals using the standard algorithm for each operation.

How this material aligns: Students multiply and divide multi-digit numbers and decimals, such as 43,560 times 7.48 and 65,170,200 divided by 800,000, using the standard algorithms this standard requires.

All materials for 6.NS.B.3 →

Download Word file

This material is in the public domain (CC0) and may be copied, edited, and shared freely, without attribution. Updated 2026-07-23.

Objectives

Students will convert reservoir storage among acre-feet, cubic feet, and gallons using stated constants.
Students will divide total storage by daily use to find how many days of water remain.

Vocabulary

Reservoir: A human-made lake, usually created by a dam, that stores water for later. Reservoirs supply cities and farms and help control floods.
Acre-foot: The amount of water it takes to cover one acre of land, about the size of a football field, one foot deep. Western states use acre-feet to measure large amounts of water.

Also: unit conversion, days of supply

Teacher overview

Large water supplies are measured in big units, and one of the most common is the acre-foot, which is enough water to cover one acre of land one foot deep. This set uses the fictional Harlow Reservoir to practice converting between acre-feet, cubic feet, and gallons, and then dividing to find how many days the stored water will last. A common misconception this set corrects is that different units mean different amounts of water; an acre-foot and its 325,851 gallons are the same water, just counted with a different-sized unit.

Harlow Reservoir: Storage, Days of Supply, and Conversions — Grade 6 Math

Name: ____________________________ Date: ______________

Problems

The fictional Harlow Reservoir holds 200 acre-feet of water when full. The town of Harlow uses 800,000 gallons of water per day. Use these conversions for every problem.

1 acre-foot = 325,851 gallons
1 acre-foot = 43,560 cubic feet
1 cubic foot = 7.48 gallons

Convert 200 acre-feet to gallons.
Convert 1 acre-foot to cubic feet, then convert that number of cubic feet to gallons using 7.48. How close is your answer to 325,851 gallons?
Using your answer from problem 1, how many days will the full reservoir last if the town uses 800,000 gallons per day and no rain refills it? Round to the nearest day.
The town wants at least a 100-day supply. How many gallons must the reservoir hold to reach that goal?
Convert the 100-day supply from problem 4 back into acre-feet. Round to the nearest acre-foot.
During a dry month the town cuts its use to 600,000 gallons per day. Starting from a full reservoir, how many days will the water last now? Round to the nearest day.
Cutting daily use from 800,000 to 600,000 gallons saves what percent of the original daily use?
Explain why measuring a large reservoir in acre-feet is more useful than counting every single gallon.

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: 65,170,200 gallons.
200 acre-feet times 325,851 gallons per acre-foot equals 65,170,200 gallons.
Answer: 325,829 gallons (about the same).
1 acre-foot is 43,560 cubic feet. 43,560 times 7.48 equals 325,828.8, which rounds to 325,829 gallons. That is almost the same as 325,851 gallons; the small difference comes from rounding in the constants.
Answer: 81 days.
65,170,200 gallons divided by 800,000 gallons per day equals 81.46, which rounds to 81 days.
Answer: 80,000,000 gallons.
100 days times 800,000 gallons per day equals 80,000,000 gallons.
Answer: 246 acre-feet.
80,000,000 gallons divided by 325,851 gallons per acre-foot equals about 245.5, which rounds to 246 acre-feet.
Answer: 109 days.
65,170,200 gallons divided by 600,000 gallons per day equals 108.6, which rounds to 109 days.
Answer: 25%.
The cut is 800,000 minus 600,000, which is 200,000 gallons. 200,000 divided by 800,000 equals 0.25, or 25 percent.
Answer: Answers will vary. Accept clear reasoning that the reservoir holds tens of millions of gallons, so a small unit like the gallon gives huge, hard-to-read numbers, while the acre-foot keeps the totals short and easy to compare.

Differentiation

Support

Give students a conversion reference strip with the three constants and a worked model of problem 1. Let them use a calculator and remind them to line up commas so the millions place stays correct.

Extension

Ask students to find how many more acre-feet the reservoir would need to guarantee a full year (365 days) of supply at 800,000 gallons per day, showing every conversion step.

ELL supports

Pre-teach "reservoir," "acre-foot," and "supply" with a labeled picture of a filled valley behind a dam. Provide a template that shows where the starting number, the conversion constant, and the operation go for each problem.

Extensions

Have students find the storage of a real reservoir near their region, often listed in acre-feet, and estimate its days of supply for a town using 800,000 gallons per day, noting that their estimate ignores rainfall and inflow.