Question 571547
Let {{{ s }}} = speed of boat in still water in mi/hr
Let {{{ c }}} = speed of current in mi/hr
{{{ s - c }}} = speed of boat going against the current in mi/hr
{{{ s + c }}} = speed of boat going with the current in mi/hr
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given:
Going upstream:
(1) {{{ d[1] = ( s - c )*t[1] }}} 
(1){{{ 15 = ( s - c )*10 }}}
Going downstream:
(2) {{{ d[2] = ( s + c )*t[2] }}}
(2) {{{ 22.5 = ( s + c )*5 }}}
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(1) {{{ s  - c = 3/2 }}}
(2) {{{ s + c =  4.5 }}}
add the equations:
(2) {{{ s + c =  4.5 }}}
(1) {{{ s - c = 3/2 }}}
{{{ 2s = 6 }}}
{{{ s = 3 }}}
and
(1) {{{ s  - c = 3/2 }}}
(1) {{{ 3  - c = 3/2 }}}
(1) {{{ -c = -1.5 }}}
(1) {{{ c = 1.5 }}}
The speed of the boat in still water is 3 mi/hr
The speed of the current is 1.5 mi/hr
check:
(1){{{ 15 = ( s - c )*10 }}}
(1) {{{ 15 = ( 3 - 1.5 )*10 }}}
(1) {{{ 15 = 1.5*10 }}}
(1) {{{ 15 = 15 }}}
and
(2) {{{ 22.5 = ( s + c )*5 }}}
(2) {{{ 22.5 = ( 3 + 1.5 )*5 }}} 
(2) {{{ 22.5 = 4.5*5 }}}
(2) {{{ 22.5 = 22.5 }}}
OK