Aman covers a distance of 15kn, in 3 hours partly by walking and partly by running. if he walks at 3km/h and runs at 9km/h, find the distance he covers by running

Speed = distance/time
distance = speed * time

let x = time covered by running
15 = run distance + walk distance
15 = 9(x) + 3(3-x)
15 = 9x + 9 - 3x
15 - 9 = 6x
6 = 6x
x = 1 hour

Therefore:
Running distance = speed * time
RD = 9km/hr * 1 hr
RD = 9km