Question 877381
Average speed is always: ( total distance ) / ( total time )
Total time = {{{ 10 + 5 = 15 }}} sec
convert sec to hrs
( sec ) x ( min/sec ) x ( hrs / min ) = hrs
{{{ 15*( 1/60 )*( 1/60 ) = 15 / 3600 }}}
{{{ 15/3600 = 1/240 }}}  hrs
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Convert {{{ 10 }}} sec to hrs
{{{ 10*( 1/3600 ) = 1/360 }}} hrs
The 1st distance is:
{{{ d[1] = 54*( 1/360 ) }}} km
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Convert {{{ 5 }}} sec to hrs
{{{ 5*( 1/3600 ) = 1/720 }}}
The 2nd distance is:
{{{ d[2] = 96*( 1/720 ) }}} km
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 ( total distance ) / ( total time ) =
{{{ ( d[1] + d[2] ) / ( 1/240 ) }}}
{{{ ( 1/360 + 1/720 ) / ( 1/240 ) }}}
{{{ ( 3/720 ) / ( 1/240 ) }}}
{{{ ( 3*240 ) / 720 }}}
{{{ 720/720 = 1 }}}
The average speed is 1 km/hr
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Check:
at average speed of 1 km/hr, how 
many km in 15 sec?
{{{ d = 1*(1/240) }}}
{{{ d =1/ 240 }}} km
{{{ 1/360 + 1/720 = 1/240 }}}
{{{ 3/720 = 1/240 }}}
{{{ 3/720 = (3/3)*( 1/240 ) }}}
{{{ 3/720 = 3/720 }}}
OK