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+=+8
