10. (12 points) Xiao Zhang drives a car from location $A$ at the foot of a mountain, passes the mountain top, to location $B$ at the foot of the mountain on the other side, and then returns along the same route. The car's speed uphill is 30 kilometers per hour, and downhill is 40 kilometers per hour. When Xiao Zhang returns to $A$, he finds that the return trip took 10 minutes less than the outbound trip, and the car's odometer increased by 240 kilometers. Xiao Zhang spent a total of $\qquad$ hours for this round trip.
【Solution】According to the analysis, the total distance is 240, so the uphill and downhill distances for a round trip are 120 each, then the time spent is: $120 \div 40 + 120 \div 30 = 7$

That is, the total time for one round trip is: 7 hours.