During the time the engine fires, there is an acceleration of the spaceship. Once, engine is off, the spaceship moves with a constant velocity.
a = ( v - u ) / t
a = (60 - 50) /10 = 1 km/s²
distance travelled in the first 10 sec. = s1 = u t + 1/2 a t² or (v + u)/2 * t
s1 = 550 km
distance travelled in 20 sec. after engine firing is over = 60 km/s * 20 s
s2 = 1, 200 km
Total distance travelled = 1, 750 km