Two vehicles meet with different start and velocity

A bus has traveled 210 miles, and continues at a rate of 50 miles per hour. A van has driven 170 miles, and continues at a rate of 60 miles per hour. In how many hours will the two vehicles have traveled the same distance?

let us say:
S1: the distance of bus
S2: the distance of van
t: the time when the vehicles have the same distance

in this problem we are asked to determine how many hours the vehicle will have the same distance, so:

S1=S2 .......(1)

S1= 210 + (50*t) ....(2)
S2=170+(60*t) ...........(3) 

because S1=S2, then: 

210 + (50*t) = 170+(60*t)
210-170 = (60*t)-(50*t)
40=10*t
thus we can get the t =4

the answer is 4 hours