Question 883918
Let {{{ s }}} = the speed of the boat in still water
Let {{{ c }}} = the speed of the current
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Going upstream:
(1) {{{ 30 = ( s - c )*3 }}}
Going downstream:
(2) {{{ 30 = ( s + c )*1.5 }}}
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(1) {{{ 3s - 3c = 30 }}}
(2) {{{ 1.5s + 1.5c = 30 }}}
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(1) {{{ s - c = 10 }}}
(2) {{{ 15s + 15c = 300 }}}
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Multiply both sides of (1) by {{{ 15 }}}
and add the equations
(1) {{{ 15s - 15c = 150 }}}
(2) {{{ 15s + 15c = 300 }}}
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{{{ 30s = 450 }}}
{{{ s = 15 }}}
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The speed of the boat in still water is 15 mi/hr
check:
(1) {{{ s - c = 10 }}}
(1) {{{ 15 - c = 10 }}}
(1) {{{ c = 5 }}}
and
(2) {{{ 30 = ( s + c )*1.5 }}}
(2) {{{ 30 = ( 15 + 5 )*1.5 }}}
(2) {{{ 30 = 20*1.5 }}}
(2) {{{ 30 = 30 }}}
OK