Question 885238
Average speed always equals 
( total distance traveled ) / ( total time spent traveling )
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Given:
Total time = {{{ 30 + 20 = 50 }}} min
In hours:
Total time = {{{ 50/60 = 5/6 }}} hrs
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For 1st part of trip:
{{{ d[1] = 40*(1/2) }}} ( 30 min = 1/2 hr )
{{{ d[1] = 20 }}} mi
For 2nd part of trip:
{{{ d[2] = 20*(1/3) }}} ( 20 min = 1/3 hr )
{{{ d[2] = 20/3 }}} mi
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Average speed = {{{ (( d[1] + d[2] )) / ( 5/6 ) }}}
Average speed = {{{ (( 20 + 20/3 )) / ( 5/6 ) }}}
Average speed = {{{ ( 80/3 )*( 6/5 ) }}}
Average speed = {{{ 160/5 = 32 }}}
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His average speed was 32 mi/hr


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