Question 581626
Let {{{ s }}} = the speed of the boat in still water
{{{ s + 5 }}} = speed of boat going downstream
{{{ s - 5 }}} = speed of boat going upstream
Let {{{ t }}} = time going both upstream and downstream 
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Going downstream:
(1) {{{ 94 = ( s + 5 )*t }}}
Going upstream:
(2) {{{ 47 = ( s - 5 )*t }}}
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(1) {{{ 94 = s*t + 5t }}}
(2) {{{ 47 = s*t - 5t }}}
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Subtract (2) from (1)
(1) {{{ 94 = s*t + 5t }}}
(2) {{{ -47 = -s*t + 5t }}}
{{{ 47 = 10t }}}
{{{ t =4.7 }}} hrs
and
(1) {{{ 94 = ( s + 5 )*4.7 }}}
(1) {{{ 94/4.7 = s + 5 }}}
(1) {{{ 20 = s + 5 }}}
(1) {{{ s = 15 }}}
The speed in still water is 15 mi/hr
check answer:
(2) {{{ 47 = ( 15 - 5 )*t }}}
(2) {{{ 47 = 10t }}}
(2) {{{ t =4.7 }}}
OK