Question 1145222
Ave speed = [ total distance traveled ] / [ total time spent traveling ]
Let {{{ s }}} = speed min in mi/hr on winding road 
{{{ s + 20 }}} = speed in mi/hr on level road
Let {{{ d }}} = distance traveled on winding road
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On winding road:
(1) {{{ d = s*3 }}}
On level road:
(2) {{{ 306 - d = ( s + 20 )*3 }}}
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(2) {{{ 306 - d = 3s + 60 }}}
(2) {{{ 3s + d = 246 }}}
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Plug (1) into (2)
(2) {{{ 3s + 3s = 246 }}}
(2) {{{ 6s = 246 }}}
(2) {{{ s = 41 }}}
{{{ s + 20 = 61 }}} mi/hr
The ave speed on the level road is 61 mi/hr
check:
(1) {{{ d = 3s }}}
(1) {{{ d = 3*41 }}}
(1) {{{ d = 123 }}} mi
(2) {{{ 306 - d = ( s + 20 )*3 }}}
(2) {{{ 306 - 123 = ( 41 + 20 )*3 }}}
(2) {{{ 183 = 61*3 }}}
(2) {{{ 183 = 183 }}}