A truck drove from Frankfort to Richmond at a constant speed. On the way back to Frankfort, the trucker drove the same speed, but on this return trip, the truck broke down after 1 hour of travel when the truck was 30 miles from Frankfort. What was the speed of the truck?
a) 15 mph
b) 30 mph
c) 45 mph
d) 60 mph

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