A square of side 6 cm is cut along a midline into two identical rectangles. What is the perimeter of one of them?
Hints
Take them one at a time. The first gives nothing away.
1A nudge
What size is each rectangle?
2The strategy
Cutting a square down the middle leaves two rectangles with one side unchanged and the other halved. Write down both side lengths before you compute anything.
3The full solution
Each rectangle is 6 by 3, so its perimeter is 2 x (6 + 3) = 18.
Solution— no shortcut on this one
Cutting a 6 by 6 square along a midline gives two rectangles measuring 6 by 3. The perimeter of one is 2 x (6 + 3) = 18 cm. Note that the two pieces together have a total perimeter of 36, more than the square's 24, because the cut created two new edges — a useful thing to notice. The transferable idea: cutting does not change area but it does change perimeter.
Why this is on the test: Cutting a shape leaves the area alone and increases the perimeter, and mixing those two up is a recurring error.