The current of a stream at 1 kmph. A motor boat goes 35 km upstream and back to the starting point in 12 hours. The speed of the motor boat in still water is_________?

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The current of a stream at 1 kmph. A motor boat goes 35 km upstream and back to the starting point in 12 hours. The speed of the motor boat in still water is_________?

A. 6 kmph
B. 7 kmph
C. 8.5 kmph
D. 8 kmph
Explanation:
S = 1
M = x
DS = x + 1
US = x – 1
35/(x + 1) + 35/(x – 1) = 12
x = 6