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?  Pick
A) 36 mph	B) 40 mph	C) 39 mph	D) 42 mph	E) 38 mph

Rationale: 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 = 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 = 4.25 hours and total distance is 90 + 75 = 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 . . . .
Answer: C