Question 509378
Let {{{ t }}} = time for the boats to be {{{ 25 }} mi apart
given:
{{{ c = 2 }}} mi/hr is the current
{{{ s }}} mi/hr  = speed of slower boat in still water
{{{ s + 5 }}} mi/hr = speed of faster boat in still water
The slower boat going upstream:
{{{ 12 = ( s - 2 )*2 }}}
{{{ 12 = 2s - 4 }}}
{{{ 2s = 16 }}}
{{{ s = 8 }}} mi/hr
---------------
Going downstream:
slower boat:
{{{ d = ( 8 + 2)*t }}}
(1) {{{ d = 10t }}}
faster boat:
{{{ d + 25 = ( 8 + 5 + 2 )*t }}}
(2) {{{ d + 25 = 15t }}}
Substitute (1) into (2)
(2) {{{ 10t + 25 = 15t }}}
(2) {{{ 5t = 25 }}}
(2) {{{ t = 5 }}}
In 5 hrs, the boats will be 25 mi apart
check answer:
(1) {{{ d = 10*5 }}}
(1) {{{ d = 50 }}} mi
and
(2) {{{ d + 25 = 15t }}}
(2) {{{ 75 = 15t }}}
(2) {{{ t = 5 }}}
OK