Question 621317
Let {{{ s }}} = the speed of the stream
Let {{{ t }}} = the time to go upstream and downstream
Going downstream:
(1) {{{ 50 = ( 36 + s )*t }}}
Going upstream:
(2) {{{ 30 = ( 36 - s )*t }}}
--------------------
(1) {{{ 50 = 36t + s*t }}}
(2) {{{ 30 = 36t - s*t }}}
Add the equations
{{{ 80 = 72t }}}
{{{ t = 80/72 }}}
{{{ t = 10/9 }}} hrs
and
(1) {{{ 50 = ( 36 + s )*(10/9) }}}
(1) {{{ 9*50 = 10*( 36 + s ) }}}
(1) {{{ 45 = 36 + s }}}
(1) {{{ s = 9 }}}
The speed of the stream is 9 mi/hr
check:
(2) {{{ 30 = ( 36 - s )*t }}}
(2) {{{ 30 = ( 36 - 9 )*(10/9) }}}
(2) {{{ 30 = 27*(10/9) }}}
(2) {{{ 270 = 270 }}}
OK