A car drives 90 miles on local roads at 30 mph, and 75 miles on the highway at 60 mph, what is the average speed of the entire trip?  Answers:
A) 36 mph
B) 40 mph
C) 39 mph
D) 42 mph
E) 38 mph
So The Answer is Plainly C....   we have a general relation for speed , time and distance : v (velocity) * t ( time)= d (distance)   for first part we have d=90 miles , and v= 30 mph so we can obtain time : 30*t = 90 Or t= 90/30= 
<gadget id="calculator">90/30</gadget>
<output>3</output>
3 hours the needed time to cover 90 miles   in the same way we should divide 75 to 60 to find the needed time to cover 75 miles ,so t= 1.25 hours   so the total time for covering total distance would be  3+1.25= 
<gadget id="calculator"> 3+1.25</gadget>
<output>4.25</output>
4.25 hours and total distance is   90+75= 
<gadget id="calculator">  90+75</gadget>
<output>165</output>
165 miles   Final stage : average speed is total distance divide to total time : 165/4.25= 38.8 miles per hour rounded up to 39 MPH....
<result>C</result>
A car drives 90 miles on local roads at 30 mph, and 75 miles on the highway at 60 mph, what is the average speed of the entire trip?