Question 728479
Average speed  = ( total distance ) / ( total time )
1st trip:
{{{ d = 40t }}}
2nd trip:
{{{ d = 80*( t - 1 ) }}}
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By sustitution:
{{{ 40t = 80*( t - 1 ) }}}
{{{ 40t = 80t - 80 }}}
{{{ 40t = 80 }}}
{{{ t = 2 }}}
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{{{ d = 40t }}}
{{{ d = 40*2 }}}
{{{ d = 80 }}}
The distance traveled is {{{ 2d }}}
{{{ 2d = 160 }}}
The total time is {{{ t + t - 1 = 2*2 - 1 }}}
{{{ 2*2 - 1 = 3 }}}
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( total distance ) / ( total time ) = {{{ 160/3 }}}
{{{ 160/3 = 53.333 }}}
The average speed is 53.333 mi/hr