Answer:
1.5 km/hour
Step-by-step explanation:
d is the David's speed in still water
c is the speed of the current
Going with the current, he travels d+c km per hour.
Going against the current, he travels d-c km per hour.
d+c = (20 km)/(2.5 hours) = 8 km/hour
d-c = (20 km)/(4 hours) = 5 km/hour
Add the equations to eliminate the c terms
d+c = 8
d-c = 5
----------
2d = 13
d = 6.5 km/hour
c = 8-d = 1.5 km/hour
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