Speed is the ratio of distance travelled to time taken. It is given by:
Speed = distance / time
Let a represent the speed of the motor boat in still water and t represent the time. Hence for downstream:
a + 5 = 80/t
t = 80/(a + 5)
For upstream:
a - 5 = 60/t
a - 5 = 60 / (80/(a + 5))
a - 5 = 0.75a + 3.75
0.25a = 8.75
a = 35 miles per hour
The speed of the motor-boat in still water is 35 miles per hour.
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