<PRE><B>Question 819800</B>
Average speed always = ( total distance ) / ( total time )
Let {{{ 2d }}} = total distance
Let {{{ t[1] }}} = time for 1st part of trip
Let {{{ t[2] }}} = time for 2nd part of trip
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For 1st part of trip:
{{{ d = 40t[1] }}}
{{{ t[1] = d/40 }}}
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For 2nd part:
{{{ d = 60t[2] }}}
{{{ t[2] = d/60 }}}
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Average speed:
{{{ 2d / ( d/40 + d/60 ) }}}
{{{ ( 2 / ( 1/40 + 1/60 ) )*( d/d ) }}}
Multiply top and bottom by {{{ 120 }}}
{{{ ( 2*120 ) / ( 3 + 2 ) }}}
{{{ 240 / 5 = 48 }}}
The average speed for the entire journey is 
48 mi/hr
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