From 9:00 P.M. to 5:00 A.M. is a tour of duty of 8 hours. Since 60 minutes = 1 hour, when the watchman checks in every 40 minutes he checks in every 40/60, or every 2/3, ofanhour.
8/1 ÷ 2/3 = 8/1 x 3/2 = 12
The watchman must check in once at the beginning of each of the twelve 40-minute periods, or 12 times.
But he must also check in at the end of the last period. Thus, he checks in 12 + 1, or 13, times.

The area of the entire piece of canvas = 10 inches × 14 inches = 140 square inches. The portion painted on equals 8 inches × 12 inches = 112 square inches. (This is determined by subtracting 1 inch from the length of each side to account for the margin.) The portion used for painting can be expressed as a fraction: 112/140. Reduce this fraction (divide 112 by 140) to determine that 80% of the canvas is covered with paint.