In a rectangle, the perimeter is 72 and one of the sides is 16. Find the area.

In any rectangle, the opposite sides are pairwise equal, therefore, its semiperimeter is equal to the sum of the lengths of two adjacent sides. The unknown side can be found as the difference between the half-perimeter and the length of the known side:
a = 72/2 – 16 = 36 – 16 = 20.
Thus, in this rectangle, one side is 16, the other is 20.
The area of the rectangle is equal to the product of the adjacent sides: S = a * b = 20 * 16 = 320.