This question can be solved by TESTing VALUES (as some of the explanations have noted). You can keep the values really small though and save some time on the calculations:
We're told that driving 1 extra hour AND at a speed that was 5 miles/hour faster (for the entire trip) would have increased TOTAL distance by 70 miles.
We originally drove for 1 hour at 60 miles/hour, then we would have traveled 60 miles.
Adding 1 extra hour and increasing speed by 5 miles/hour would give us.....
2 hours at 65 miles/hour, which gives us a total distance of 130 miles (which is 70 miles MORE than originally traveled).
So now we we've established the starting time, speed and distance, so we can answer the given question:
How many MORE miles would be traveled if the original time was increased by 2 hours AND the original speed was increased by 10 miles/hour?
1+2 = 3 hours
60 + 10 = 70 miles/hour
3 hours at 70 miles/hour = 210 miles
Since we originally traveled 60 miles, the 210 total miles is 150 miles MORE than originally traveled.
GMAT assassins aren't born, they're made,