Year 3 practice in finding a missing side when the perimeter is known. Page 1 opens with a worked example in three steps, using a rectangle with a perimeter of 16 cm and a top side of 6 cm. Pupils then find the missing side of three shapes. Page 2 has three more shapes and one word problem about a fence round a vegetable patch. Page 3 is the answer key, with the working for every answer. It follows England National Curriculum measurement for Year 3.
Measurement
“measure the perimeter of simple 2-D shapes”
Pupils use the perimeter of a rectangle, a square and two kinds of triangle to work back to a side they cannot see.
Number - addition and subtraction
“solve problems, including missing number problems, using number facts, place value, and more complex addition and subtraction”
Each question is a missing number problem. Pupils subtract the known sides from the perimeter, and the rectangles also need a division by 2.
A pink box shows a rectangle with a perimeter of 16 cm and a top side of 6 cm. Three steps double the 6 cm, take it from 16 cm, then share the 4 cm that is left between the two short sides.
Six shapes, each with its perimeter written inside and one side marked with a question mark: rectangles of 18, 20 and 30 cm, a square of 24 cm, a triangle of 15 cm with sides of 4 and 6 cm, and an equilateral triangle of 21 cm.
Sam's rectangular vegetable patch has a perimeter of 22 m and long sides of 8 m. Pupils work out the short side in a working box and write it in the answer sentence.
Each answer has its working beside it, such as 20 - 6 = 14 cm and 14 ÷ 2 = 7 cm for question 4.
Introducing the method
Work the example in the pink box on the board, then cover the steps and ask pupils to say what comes next.
Guided group
Give questions 2 and 5 to pupils who find subtraction hard. Dividing the perimeter by 4 or by 3 is a gentler start than the rectangle steps.
Plenary
Use the word problem as a quick check. A pupil who writes 6 m has stopped before the last step and forgotten to share the 6 m between two sides.
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