As we see that the velocity is positive for the first 50 seconds of journey therefore net displacement=net distance traveled.

calculating displacement for first 50 seconds from we get 900 meters.

then assuming that it travels for t more seconds at 30 m/sec therefore displacing 30t meters.

on solving:

t=190 seconds....

so net time time is 240 seconds from the starting place to get an avearge speed of 27.5m/sec