Question 1028108
Let {{{ d }}} = the distance in km from bottom to top of hill
Let {{{ t }}} = time in hours to go up the hill
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Convert  1 hr 20 min to hrs:
{{{ 20/60 = 1/3 }}}
{{{ 1 + 1/3 = 4/3 }}} hrs
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{{{ 4/3 - t }}} = time in hrs to go down the hill
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Equation for going up the hill:
(1) {{{ d = 12t }}}
Equation for going down the hill:
(2) {{{ d = 48*( 4/3 - t ) }}}
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By substitution:
(2) {{{ 12t = 48*( 4/3 - t ) }}}
(2) {{{ 12t = 16*4 - 48t }}}
(2) {{{ 12t = 64 - 48t }}}
(2) {{{ 60t = 64 }}}
(2) {{{ t = 16/15 }}}
and
(1) {{{ d = 12t }}}
(1) {{{ d = 12*( 16/15 ) }}}
(1) {{{ d = 12.8 }}} 
The distance in km from bottom to top of hill
is 12.8 km
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check:
(2) {{{ d = 48*( 4/3 - t ) }}}
(2) {{{ d = 48*( 4/3 - 16/15 ) }}}
(2) {{{ d = 48*( 20/15 - 16/15 ) }}}
(2) {{{ d = 48*( 4/15 ) }}}
(2) {{{ d = 12.8 }}} km
OK