Question 829340
Let {{{ s }}} = his speed paddling in still water
Let {{{ c }}} = the speed of the current
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Paddling upstream:
(1) {{{ 8 = ( s - c )*2 }}}
Paddling downstream:
(2) {{{ 16 = ( s+c )*2 }}}
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(1) {{{ 2s - 2c = 8 }}}
(2) {{{ 2s + 2c = 16 }}}
Add the equations
{{{ 4s = 24 }}}
{{{ s = 6 }}}
and
(1) {{{ 2s - 2c = 8 }}}
(1) {{{ s - c = 4 }}}
(1) {{{ 6 - c = 4 }}}
(1) {{{ c = 2 }}}
Alvin paddled 6 mi/hr in still water 
and the speed of the current is 2 mi/hr
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check:
(2) {{{ 16 = ( 6+2 )*2 }}}
(2) {{{ 16 = 16 }}}
and
(1) {{{ 8 = ( 6 - 2 )*2 }}}
(1) {{{ 8 = 8 }}}
OK