Grimsholt Settling Basins: A Grade 5 Missing Dimension Lesson

Hook (5 min): Sketch a rectangle on the board labeled 10 feet by 8 feet and tell the class it is the floor of a settling basin at Grimsholt. Ask how they could figure out the volume, then ask what is missing from the drawing. Collect answers until someone names the depth, and write the question the village is actually asking: how deep should we dig?

Direct instruction (12 min): Model the forward calculation first. A basin 10 feet long, 8 feet wide, and 4 feet deep gives 10 x 8 = 80, and 80 x 4 = 320 cubic feet. Then reverse it on the same drawing: if the basin must hold 320 cubic feet and the floor is 10 feet by 8 feet, divide 320 by 80 to get a depth of 4 feet. Write both sentences side by side so students see the same three numbers in a multiplication and in a division. Close by introducing the conversion that plant operators use, 7.48 gallons in every cubic foot, and convert 320 cubic feet to 2,393.6 gallons.

Guided practice (10 min): In pairs, students work a second reversal with you circulating. A basin must hold 600 cubic feet and the site allows a floor 20 feet long and 5 feet wide. Pairs find 20 x 5 = 100 square feet of floor, then 600 divided by 100 = 6 feet deep. As a formative check, ask each pair to hold up fingers for the depth they found; if more than a quarter of the class shows a number other than 6, work a third reversal together before moving on.

Independent practice (13 min): Distribute the student handout and have students complete questions 1 through 5 on their own. Circulate and watch for students who divide by only one dimension instead of by the product of the two known dimensions, which is the most common error at this stage.

Closure and exit ticket (5 min): Students answer handout question 6 on an index card and hand it in on the way out. Collect the cards and sort them into correct and incorrect piles before the next lesson so you know who needs the support activity.