tom traveled the entire 100 miles trip . if he did the first 50 miles of at a constant rate 20 miles per hour and the remaining trip of at a constant rate 50 miles per hour , what is the his average speed , in miles per hour ?
"avg speed = total distance / total time = ( d 1 + d 2 ) / ( t 1 + t 2 ) = ( 50 + 50 ) / ( ( 50 / 20 ) + ( 50 / 50 ) ) = 60 * 2 / 3 = 28.57 mph c"
divide(100, add(divide(50, subtract(100, 50)), divide(50, 20)))