Equations
The distance x traveled by an object moving at a speed v during a time t is given by:
x = v.t
The bus left San Diego at 1:30 at a speed of 50 mph. A train left San Diego at 3:30 in the same direction at 70 mph.
Note the train left 2 hours later than the bus. This allowed the bus to travel a distance of:
x = 50 * 2 = 100 mi
So the bus is 100 miles ahead of the train at 3:30 pm.
Now let's assume that t more hours pass.
The bus will travel a distance of
xb = 50.t
The train travels a distance of:
xt = 70.t
For the train to catch up with the bus, its distance must be equal to the distance of the bus plus the 100 miles it's ahead of the train, thus:
70.t = 50.t + 100
When we solve this equation for t, we will know the number of hours needed. Subtracting 50.t:
70.t - 50.t = 100
Operating:
20.t = 100
Dividing by 20:
t = 100/20
t = 5
This means that 5 hours must pass for the train to catch up with the bus. Since the train left at 3:30, that will happen at 8:30