Question 27210
Let 'x' be the time you want to know. Therefor:

40[m/h].x[h] + 5[m] = 55[m/h].x[h]
This means that after a certain time 'x', the 1st car has travelled a 
distance 40[m/h].x[h] The question was the exact time at which the 2nd car was 5 miles ahead. So you take the distance of the first car 40[m/h].x[h], and add the 5 miles. This should be equal to the distance of the 2nd car (55[m/h].x[h])

=> 40x+5=55x
=> 5 = 15x
=> x = 1/3 or in this case, 1/3 of an hour, being 20 minutes.