Let x = rate of the current
Speed of boat upstream (against current) = 30 - x
Speed of boat downstream (with current) = 30 + x
Distance = (rate)(time), so time = (distance)/(rate)
Time upstream = time downstream
45/(30 - x) = 63/(30 + x)
Cross multiplying, we have: 45(30 + x) = 63(30 - x)
1350 + 45x = 1890 - 63x
108x = 540
x = 5
ANSWER: The speed of the current is 5 mph.