The area of a rhombus is $240{\text{cm}}^2$ and one of its diagonals is 16 cm. Find its perimeter in cm.

½ × $16\times d$₂ = 240 gives $d$₂ = 30 cm. The half-diagonals are 8 cm and 15 cm, so each side = √(64 + 225) = √289 = 17 cm. Perimeter = $4\times 17=68$ cm.