What is the perimeter of a rectangle that is 9 cm long and 4 cm wide?

Perimeter is the total distance all the way around the shape, so you must include every side, not just two of them. A rectangle has four sides: two lengths and two breadths. The length 9 cm appears twice and the breadth 4 cm appears twice. So the perimeter is 9 + 4 + 9 + 4 = 26 cm. Your answer of 13 was just one length plus one breadth, which is only half the way around. A shorter formula is Perimeter = 2 × (length + breadth) = 2 × (9 + 4) = 2 × 13 = 26 cm. So your friend is right at 26 cm. The doubling step is the part that is easy to forget, so always remember both pairs of sides.