Worksheet Grade 6 Math

Kettle Ridge Microbe Basin: BOD Removal Percents and Ratios

A grade 6 math worksheet where students use a fictional plant's microbe basin readings to find BOD removal percents, a feeding ratio, and decimal means.

Grade: Grade 6
Subject: Math
Material: Worksheet
Time: 30 min
Prep: No prep
Materials: Calculator (optional)

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: Items 2, 4, and 7 have students find a percent of a quantity and scale a 3-to-1 feeding ratio, the ratio and percent reasoning this standard targets.

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: Items 8 and 9 require students to add three multi-digit decimal BOD readings and divide the sum by three, the exact decimal fluency this standard expects.

All materials for 6.NS.B.3 →

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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 calculate the percent of waste that microbes remove from water using a removal-efficiency formula.
Students will add and average multi-digit decimal readings and scale a simple whole-number ratio.

Vocabulary

Key words: microbe, removal efficiency, ratio, mean

Teacher overview

Some water plants clean wastewater with living microbes instead of chemicals alone. Using living things to treat water is called environmental biotechnology. In an aeration basin, billions of tiny microbes eat the organic waste carried in the water. Operators measure that waste as BOD, short for biochemical oxygen demand, in milligrams per liter. Kettle Ridge is a fictional plant created for this worksheet. Students find how much of the waste the microbes remove, using the removal-efficiency percent that operators calculate every day. A common error is dividing by the ending reading instead of the starting one, so remind students to divide the amount removed by the amount that came in.

Kettle Ridge Microbe Basin: BOD Removal Percents and Ratios — Grade 6 Math

Name: ____________________________ Date: ______________

Student Worksheet

Suppose the fictional Kettle Ridge plant cleans dirty water with tiny microbes. The water comes in with a BOD of 240 mg/L. After the microbes eat the waste, the BOD is 30 mg/L. Use these two numbers below. The removal percent equals the amount removed, divided by the amount that came in, times 100.

Subtract to find how much the BOD dropped from the start to the end.
Find the removal percent. Divide your drop by 240, then multiply by 100.
The plant must remove at least 85 percent of the waste. Does Kettle Ridge pass?
A second basin comes in at 200 mg/L and leaves at 30 mg/L. Find its removal percent.
Which basin removed a higher percent of the waste?
The basin holds 3 pounds of microbes for each 1 pound of waste eaten daily. Write this as a ratio.
One day the microbes eat 4 pounds of waste. Using the 3-to-1 ratio, how many pounds of microbes match that?
Three daily BOD readings are 31.5, 28.0, and 30.5 mg/L. Add them.
Find the mean of the three readings. Divide your sum by 3.
The limit is 30 mg/L. Is the mean at or below the limit?

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: 210 mg/L. The BOD dropped from 240 to 30, and 240 − 30 = 210 mg/L removed.
Answer: 87.5%. 210 ÷ 240 = 0.875, and 0.875 × 100 = 87.5 percent.
Answer: Yes. 87.5 percent is above the 85 percent rule, so the plant passes.
Answer: 85%. First 200 − 30 = 170 removed; then 170 ÷ 200 = 0.85, and 0.85 × 100 = 85 percent.
Answer: The first basin. It removed 87.5 percent, while the second removed 85 percent.
Answer: 3 to 1. Three pounds of microbes for each one pound of waste is written 3:1.
Answer: 12 pounds. At the 3-to-1 ratio, 4 pounds of waste × 3 = 12 pounds of microbes.
Answer: 90.0 mg/L. 31.5 + 28.0 + 30.5 = 90.0 mg/L.
Answer: 30.0 mg/L. 90.0 ÷ 3 = 30.0 mg/L.
Answer: Yes. The mean 30.0 mg/L is exactly at the 30 mg/L limit, so it is at or below it.

Differentiation

Support

Give students the two starting numbers, 240 and 30, already written at the top of the page, and work item 1 out together as a model. Provide the removal rule on a printed card: amount removed, divided by amount in, times 100. Allow a calculator for the division steps so the focus stays on choosing the right numbers.

Extension

Ask students to design a third basin that removes exactly 90 percent of its waste. They pick a whole-number starting reading, then work backward to find the ending reading that produces 90 percent removal, and show the calculation that proves it.

ELL supports

Pre-teach the words microbe, waste, remove, and mean with a simple labeled picture of a basin. Pair each word with a short phrase in the student's home language where possible. Allow students to answer with labeled number sentences instead of full written sentences.