Question 619795
Let t be the time in hours the second bus is on the road.
Then {{{t+ 2}} is time the first bus is on the road.
When the second bus catches up to the first one, they bout will have traveled the same distance.
{{{Distance = speed * time}}} 
Since the distance is the same for both, you can equate 
{{{Distance1 = Distance2}}}
{{{Speed1 * Time1 = Speed2 * Time2}}}
{{{40 * (t+2) = 60 * t}}}
{{{40t + 80 = 60t}}}
{{{80 = 20t}}}
{{{4 = t}}}
So the second bus will catch up in 4 hours.

Another way to think thru this problem is to realize the first bus had a 2 hour head start. At 40 mph, the first bus is 80 miles ahead of the second bus.
The two buses are traveling at a difference in speed of 20mph.
How many hours will it take to catchup 80 miles at 20mph? Again the answer is 4 hours. 
Always more than one way to look at a math problem. :)