The speed of a stream is 4 mph. A boat travels 10 miles upstream in the same time it takes to travel 18 miles downstream. What is the speed of the boat in still water? The speed is
mph.

This is an RTD question. Here is a table based on the given information:

up/down . . . .rate . . . .time . . . .distance

upstream . . . . r-4 . . . . T . . . . 7

downstream . . . . r+4 . . . . T . . . .15

Now, rate x time = distance, so

time = distance / rate.

solve each for T and we get

T = (7/(r-4))

T = (15/(r+4))

set the times equal and we get

7%2F%28r-4%29+=+15%2F%28r%2B4%29

cross multiply to get

7r+%2B+28+=+15r+-+60

subtract 7r to get

28+=+8r+-+60

add 60 to both sides to get

88+=+8r

divide by 8 to get

r+=+8tep