Answer:
 40 mph
Step-by-step explanation:
We assume "outbound" refers to the trip to the lake. The ratio of speeds is inversely proportional to the ratio of times, so ...
 outbound speed : inbound speed = 4 : 3
These differ by one ratio unit, so that one ratio unit corresponds to the speed difference of 10 mph. Then the 4 ratio units of outbound speed will correspond to ...
 4×10 mph = 40 mph
Paul's average speed on the outbound trip was 40 mph.
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The distance to the lake was 120 mi.
Answer:Paul's average speed on the outbound trip is 40mph
Step-by-step explanation:
Let x represent Paul's outbound trip which is the trip to the lake.
The trip to the lake took 3 hours.
Distance travelled = speed × time
It means that
Distance covered on the trip to the lake would be
3 × x = 3x
the trip back took 4 hours. He averaged 10 mph faster on the trip there than on the return trip. It means that his speed would be
x - 10
Therefore, distance travelled on return trip would be
4(x - 10) = 4x - 40
Since the distance travelled is the same, it means that
3x = 4x - 40
4x - 3x = 40
x = 40
