Question 720809
Let {{{ s }}} = the speed of the boat in still water
Let {{{ c }}} = the speed of ther current
{{{ s - c }}} = the speed of the boat going against the current
{{{ s + c }}} = the speed of the boat going with the current
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Going upstream:
(1) {{{ 4 = ( s - c )*(2/5) }}}
Going downstream:
(2) {{{ 4 = ( s + c )*(1/5) }}}
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(1) {{{ 20 = 2s - 2c }}}
(1) {{{ 10 = s - c }}}
and
(2) {{{ 20 = s + c }}}
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Add (1) and (2)
{{{ 30 = 2s }}}
{{{ s = 15 }}}
and
(1) {{{ 10 = s - c }}}
(1) {{{ 10 = 15 - c }}}
(1) {{{ c = 15 - 10 }}}
(1) {{{ c = 5 }}}
The speed of the boat in still water is 15 mi/hr
The speed of ther current is 5 mi/hr
check:
(2) {{{ 4 = ( s + c )*(1/5) }}}
(2) {{{ 4 = ( 15 + 5 )*(1/5) }}}
(2) {{{ 4 = 20/5 }}}
(2) {{{ 4 = 4 }}}
OK
(1) {{{ 4 = ( s - c )*(2/5) }}}
(1) {{{ 4 = ( 15 - 5 )*(2/5) }}}
(1) {{{ 4 = 20/5 }}}
(10 {{{ 4 = 4 }}}
OK