Find speed of the boat in still water

Question

A boat travels 50 miles downstream in the same time that it takes to go 30 miles upstream. The speed of the stream is 3 miles per hour. Use algebra to find the speed of the boat in still water.

Answer 1

Let v = speed of boat in still water.

Therefore in time t we get

30 = t (v-3) for upstream

50 = t (v+3) for downstream

since t is the same we can write

30/(v-3) = 50 /(v+3)

50v-150 = 30v + 90

20 v = 240

v= 12 mph