Question 263127
This is an RTD question. Here is a table based on the given information:
up/down . . . . . . . . . . .rate . . . . . . . . . . .time . . . . . . . . . .distnace
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/(r-4) = 15/(r+4)}}}
cross multiply to get
{{{7r + 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}}}