Question 1107709
Let {{{ s }}} = speed of boat in still water in mi/hr 
Let {{{ c }}} = speed of current in mi/hr
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Going upstream:
(1) {{{ 30 = ( s - c )*6 }}}
Going downstream:
(2) {{{ 72 = ( s + c )*6 }}}
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(1) {{{ s - c = 5 }}}
and
(2) {{{ s + c = 12 }}}
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Add the equations
{{{ 2s = 17 }}}
{{{ s = 8.5 }}}
and
(1) {{{ 8.5 - c = 5 }}}
(1) {{{ c = 3.5 }}}
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The speed of boat in still water is 8.5 mi/hr
The speed of the current is 3.5 mi/hr