Answer:
4 mph
Step-by-step explanation:
The speed downstream is (28 mi)/(2 h) = 14 mph.
The speed upstream is (18 mi)/(3 h) = 6 mph.
The speed of the current is half the difference of these:
(14 mph -6 mph)/2 = 4 mph
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The current adds to the downstream speed and subtracts from the upstream speed, so the difference of those speeds is twice the current's speed.
