If Phong paints 1 square yard in 2 minutes,how long will it take him to paint a fence that is 3 feet tall and 30 feet long?
1 yd = 3 ft
1 yd ^ 2 = ( 3 ft ) ^ 2 = 9 ft ^ 2
1 yd ^ 2 / 2 min = 9 / 2 = 4.5 ft ^ 2 / min
3 ft * 30 ft = 90 ft ^ 2
90 ft ^ 2 / ( 4.5 ft ^ 2 / min ) = 20 min