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Classroom activity Grade 3 Math

Willowbrook's Wetland Filter Relay: A Two-Step Word Problem Activity

A grade 3 relay activity where teams carry pollutant tokens through a model constructed wetland and solve two-step subtraction problems about what remains.

Objectives

  1. Students will subtract a sequence of amounts from a starting total and record the remaining amount after each step.
  2. Students will write and solve a two-step equation using a letter for an unknown quantity.
  3. Students will estimate an answer before calculating and compare the two.

Vocabulary

Key words: constructed wetland, pollutant, estimate, equation

Teacher overview

At the fictional Willowbrook Elementary, the class built a model constructed wetland: a shallow tray layered with gravel, sand, and plants, standing in for the kind of wetland that some communities build to help filter stormwater before it reaches a creek. In this relay, teams carry a starting cup of "pollutant tokens" through three zones that each remove a set number of tokens, then solve two-step subtraction problems about what happens along the way. A common misconception this activity addresses directly is the assumption that a filter must remove everything that enters it; the relay's own numbers show that some tokens still make it all the way through, which is also true of real wetlands.

Activity Directions

  1. Before class, set up three stations in a row, each with a collection bin and a sign: Gravel Zone (removes 4 tokens), Sand Zone (removes 3 tokens), and Root Zone (removes 2 tokens). At the start of class, explain that a constructed wetland is a shallow basin of gravel, sand, and plants that some communities build to filter pollutants out of stormwater before it reaches a creek, and that this relay acts out that filtering process.

  2. Divide the class into teams of 3 or 4 students. Give each team a cup holding 10 small paper "pollutant tokens," representing dirt, oil, and litter carried by stormwater, plus a recording sheet.

  3. Teams start at the Gravel Zone. Following the sign, they remove 4 tokens from their cup, place them in the zone's collection bin, and record how many tokens remain in their cup as question 1 on the recording sheet.

  4. Teams move to the Sand Zone, remove 3 more tokens per the sign, place them in that zone's bin, and record their new remaining total as question 2 on the recording sheet.

  5. Teams move to the Root Zone, remove 2 more tokens per the sign, place them in that zone's bin, and record their new remaining total as question 3 on the recording sheet. Whatever tokens remain in the cup at this point are the tokens that reach the creek.

  6. Once every team has passed through all three zones, teams complete questions 4 and 5 on the recording sheet, either using their own totals or as a whole-class discussion if every team's totals matched.

  7. End the activity once every team has passed through all three zones and finished the recording sheet. Gather the class at the Root Zone's collection bin and lead the debrief using question 6 on the recording sheet.

Willowbrook's Wetland Filter Relay: A Two-Step Word Problem Activity — Grade 3 Math
Name: ____________________________ Date: ______________

Student Handout

Recording Sheet: Wetland Filter Relay

Your team starts with 10 pollutant tokens in the cup. Write your team's answer next to each number as you move through the zones.

  1. At the Gravel Zone, you remove 4 tokens and place them in the bin. How many tokens are left in your cup?

  2. At the Sand Zone, you remove 3 more tokens. How many tokens are left in your cup now?

  3. At the Root Zone, you remove 2 more tokens. How many tokens are left in your cup, and how many reach the creek?

  4. A different team lost 4 tokens at the Gravel Zone and 3 tokens at the Sand Zone. They had 3 tokens left before the Root Zone. Let the letter n stand for the number of tokens that team started with. Write an equation for their relay, then find n.

  5. Estimate before you add exactly: about how many tokens does the wetland remove across all three zones, and about how many tokens should reach the creek? Then compare your estimate with your exact answer from question 3.

  6. Talk with your team, then write your idea: why does the wetland let 1 token through instead of removing all 10, and what does that 1 token stand for?

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: 6 tokens.
    Starting with 10 tokens and removing 4 at the Gravel Zone: 10 − 4 = 6 tokens remain.

  2. Answer: 3 tokens.
    Starting from 6 tokens after the Gravel Zone and removing 3 at the Sand Zone: 6 − 3 = 3 tokens remain.

  3. Answer: 1 token.
    Starting from 3 tokens after the Sand Zone and removing 2 at the Root Zone: 3 − 2 = 1 token remains and reaches the creek.

  4. Answer: 10.
    A team started with n tokens, lost 4 at the Gravel Zone and 3 at the Sand Zone, and had 3 tokens remaining before the Root Zone. The equation is n − 4 − 3 = 3, so n = 3 + 4 + 3 = 10 tokens.

  5. Answer: About 9 tokens removed and about 1 token reaching the creek, which matches the exact answer of 1 token from question 3.
    Estimating first: losing about 4, then 3, then 2 tokens from 10 is close to losing 9 tokens in all, leaving about 1. The exact answer from item 3 (1 token) matches this estimate.

  6. Debrief question: "Why does the wetland let 1 token through instead of removing all 10, and what does that 1 token stand for?" Answer: A natural filter like a constructed wetland reduces the amount of pollution passing through it, but it does not remove every single particle; the 1 remaining token stands for pollution that still reaches the creek, which is why communities also work on preventing pollution before it enters the stormwater.

Differentiation

Support (below grade level)

Provide a recording sheet with the subtraction already set up as fill-in-the-blank number sentences (10 − 4 = ___, ___ − 3 = ___, ___ − 2 = ___) so students only need to compute one step at a time. Allow teams to count tokens aloud together at each zone before writing the number.

Extension (above grade level)

Ask students to design a fourth zone that would remove exactly enough tokens so that 0 tokens reach the creek, using the totals already recorded, and write the equation that proves their answer works.

ELL supports

Pre-teach "remove," "remain," and "zone" using the physical stations themselves: point to each bin while saying the word, and post a picture card with a token count at each zone. Allow teams to answer the debrief question with a partner in their home language before sharing an English summary with the class.