Two boys, Shawn and Curtis, went for a walk. Shawn began walking 20 seconds earlier than Curtis. Shawn walked at a speed of 5 feet per second. Curtis walked at a speed of 6 feet per second. For how many seconds had Sean been walking at the moment on the two boys had walked exactly the same distance?

Distance = speed * time

for curtis d = st = 6t (where t is the time that he walks)

sean walks 20 seconds longer than curtis so we have the equation

d = s (t+20) = 5 (t+20)

as the distances are the same we have

So sean has walked 100+20 = 2

6t = 5 (t+20)

6t = 5t + 100

t = 100

so Sean walks 120 seconds when they had both walked the same distance.