(b) You have the speed from 0 to 10 seconds, it doesn't move from 10 to 15, and you have its speed from 15 to 20. You should be able to calculate the total distance travelled by considering each 3 cases separately. For example, in the first 10 seconds, if it travels at the speed of x m/s, then the distance travelled is $x \cdot 10$ metres (since $d = vt$).