Question 753567
Average speed is always ( total distance traveled ) / ( total time )
Let {{{ d }}} = total distance traveled
Let {{{ 2t }}} = total time 
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Half the time is spent traveling at {{{ 50 }}} km/hr
{{{ d[1] = 50t }}}
Half the time is spent traveling at {{{ 82 }}} km/hr
{{{ d[2] = 82t }}}
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{{{ d = d[1] + d[2] }}}
{{{ d = 50t + 82t }}}
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The average speed is
{{{ d / ( 2t ) = ( 50t + 82t ) / ( 2t ) }}}
The {{{t}}}'s cancel
Average speed = 132 / 2
Average speed = 66 km/hr