Help please?

Question

the speed of a stream is 4 mph. a boat travels 4 miles upstream in the same time it takes to travel 12 miles downstream. what is the speed of the boat in still water?

Answer 1

Let, s =  speed of the boat in still water.

Then,

speed of the up stream(in relation to the land) = s - 4.

speed of the down stream(in relation to the land) = s + 4.

Recall the formula: Time = distance/speed.

Time down stream = time up stream

12/(s + 4) = 4/(s - 4)

Cross multiplication.

12(s - 4) = 4(s + 4)

12s - 48 = 4s + 16

12s - 4s = 16 + 48

8s = 64

⇒ s = 8mph.

Therefore, the spead of the boat in still water is 8mph.