Question 1047602
Let {{{ s }}} = Kareem's speed in the water 
with no current in km/hr
Let {{{ t }}} = his time in hrs to swim
both against and with the current
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Kareem's speed going against the current
(1) {{{ 5 = ( s - 2 )*t }}}
Kareem's speed going with the current
(2) {{{ 15 = ( s + 2 )*t }}}
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(1) {{{ t = 5/( s - 2 ) }}}
and
(2) {{{ 15 / ( s + 2 ) = t }}}
by substitution:
(2) {{{ 15/( s + 2 ) = 5/( s - 2 ) }}}
(2) {{{ 15*( s - 2 ) = 5*( s + 2 ) }}}
(2) {{{ 15s - 30 = 5s + 10 }}}
(2) {{{ 10s = 40 }}}
(2) {{{ s = 4 }}}
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Kareem can swim 4 km/hr with no current
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check:
(1) {{{ 5 = ( 4 - 2 )*t }}}
(1) {{{ 5 = 2t }}}
(1) {{{ t = 5/2 }}}
and
(2) {{{ 15 = ( 4 + 2 )*t }}}
(2) {{{ 15 = 6t }}}
(2) {{{ t = 5/2 }}}
OK