Hi REINE,
This question can be solved with a mix of arithmetic and TESTing THE ANSWERS.
We're told that Adam drove 40 mph for 30 minutes. That distance = (40 mph)(1/2 hour) = 20 miles. We're then told that Beth's trip was 5 miles LONGER and 20 minutes MORE to complete - so she traveled for 25 miles and drove for 50 minutes. We're asked for Beth's average speed.
Beth drove for ALMOST an hour; if she had driven for an hour, then she would have an average speed of 25 mph. Since she drove that distance in a little LESS than an hour though, she would have driven a little faster than 25 mph.
Let's TEST Answer B: 30 mph
If she drove 30 mph for 50 minutes, then the total distance traveled would have been (30 mph)(5/6 hour) = 150/6 = 25 miles. This is an exact match for what we were told, so this MUST be the answer.
Final Answer: B
GMAT assassins aren't born, they're made,
Rich
