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A motorboat goes 30 km downstream and comes back in a total of 4.5 hours. The stream flows at 5 km/h. Which equation gives its still-water speed b?

Competition practiceBoats & Streams

A $30/ (b + 5) + 30/ (b - 5) = 4.5$
B $60/ b = 4.5$
C $30/ (b + 5) - 30/ (b - 5) = 4.5$
D $30/ (b + 5) + 30/ (b + 5) = 4.5$

Step-by-Step Solution

Downstream time 30/(b + 5) plus upstream time 30/(b − 5) equals 4.5 h (it gives b = 15).

Answer: A)

30/ (b + 5) + 30/ (b - 5) = 4.5

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