3rd Grade Missing Side Length Perimeter Worksheet: Find a, b, t

The perimeter is the distance all the way around a shape. This 3rd grade missing side length perimeter worksheet turns that idea around. Each shape already shows its total perimeter, but one side has a letter instead of a length. The first shape is a rectangle measured in feet, and the second one is a trapezoid with its sides measured in kilometres. The third is an L shape in millimetres, with six sides. So your child adds the sides they know and compares that sum with the perimeter. The difference is the missing side. Page two is the answer key, and it writes out every step, from the sum of the sides to the final answer.

Three answers to check

In the rectangle, 4 + 13 + 4 = 21 ft, and 34 − 21 = 13, so a = 13 ft. That also matches the top side, because opposite sides of a rectangle are equal. In the trapezoid, the three known sides add up to 12 km, so b = 20 − 12 = 8 km. In the L shape, the five known sides make 42 mm, so t = 56 − 42 = 14 mm.

The L shape is drawn with care. Its two bottom pieces, 7 mm and 7 mm, make the 14 mm top. In the same way, 6 mm and 8 mm on the right make the 14 mm left side. All three answers in the key are right.

At the table: a missing addend, not algebra

The key says "subtract 21 ft from both sides," which is the language of equations. Third graders do not need it. Instead, read each item as a missing addend: 21 plus what makes 34? Your child can count on from 21, or subtract on a number line. Both ways give 13.

Ask your child to point to each side with a finger as they add it, so that no length is counted twice or left out. On the L shape, it is easy to skip a side, since there are six of them. A small tick next to each length helps. This skill is part of the grade 3 measurement standard, so the page is right on level.

After the page: a string around the table

Take a long piece of string and wrap it once all the way around the edge of a book or a placemat on the table. Then measure the string with a ruler, and that length is the perimeter. Next, measure three sides only and ask your child to work out the fourth from the string. Finally, check it with the ruler. For more practice, draw a rectangle, write its perimeter inside and hide one side under a sticky note.