The area of the rectangle is 720 cm2. Find the sum of the lengths of its sides if the length of the rectangle is 80 cm.

1) Find the width of the rectangle, knowing the length and area. Let’s write the formula for the area of a rectangle:
S = a × b,
where a is the length of the rectangle, b is the width.
Let us express the width, b from the formula:
b = S: a.
Let’s calculate:
b = 720: 80 = 9 cm.
2) The sum of the lengths of all sides of a geometric figure is the perimeter. The perimeter of a rectangle is the sum of the length and width of the rectangle multiplied by 2. Let’s write the formula:
P = (a + b) × 2,
Let’s make the calculations:
P = (80 + 9) × 2 = 89 × 2 = 178 cm.
Answer: the sum of the lengths of the sides of the rectangle (perimeter) is 186 cm.