A rectangle has four sides. We know that two of those sides are 10 inches long. Had what mean that those two sides are 20 inches long. That leaves 40 inches of perimeter left. If we divide that by two we get 20. That means that the two remaining sides are 20 inches long. To check your answer add each side together.
20+20+10+10=60
The rectangle is 20 inches long.