Need help on this word problem

Question

Warren drives his car 90 miles and has a mean of a certain speed. If the mean speed had been 4 mph more, he could have traveled 102 miles in the same length of time. What was his mean speed?

Answer 1

Let S be the mean speed  

Then S + 4 is the increased speed 

Let T be the time (same in both cases) 

 

Distance = Speed times Time  so  

 

  90 = S*T                            for the 1st case  

102 = (S+4)*T = S*T + 4*T  for the 2nd case  

 

Subtract the 1st equation from the 2nd to get 

 

12 = 4*T

T = 3  hours  

 

So S = 90/T = 90/3 = 30 mph 

 

Check:  (S+4)*T = 34*3 = 102 miles