Speed at the beginning of the 5 sec . . . 20 m/s
In 5 sec, the body gains (3 x 5) = 15 m/s of speed
Speed at the end of the 5 sec . . . (20 + 15) = 35 m/s
Average speed during the 5 sec . . . (1/2) (20 + 35) = 27.5 m/s
Moving for 5 sec at the average speed of 27.5 m/s,
distance = (5 x 27.5) = 137.5 meters