Question 943456
How much of a head start did the 1st car get?
You have to convert {{{ 20 }}} sec to hours
( seconds ) x ( minutes / second ) x ( hours / minute ) = hours
{{{ 20*( 1/60 )*( 1/60 ) = 20/3600 }}}
{{{ 20/3600 = 1/180 }}} hrs
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Let {{{ d[1] }}} = 1st car's head start in miles
{{{ d[1] = r*t }}}
{{{ d[1] = 15*( 1/180 ) }}}
{{{ d[1] = 5/60 }}} 
{{{ d[1] = 1/12 }}} mi
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If you start 2 stopwatches when the 2nd car leaves,
and you time both cars, then you let {{{ t[1] }}} = 
1st car's time in hrs, and {{{ t[2] }}} = 2nd car's 
time in hrs
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2nd car's equation:
{{{ 1/4 = 30t[2] }}}
{{{ t[2] = 1/120 }}} hrs
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1st car's equation:
{{{ 1/4 - 1/12  = 15t[1] }}}
{{{ 3/12 - 1/12 = 15t[1] }}}
{{{ 15t[1] = 1/6 }}}
{{{ t[1] = 1/90 }}} hrs
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{{{ 1/120 }}} is smaller than {{{ 1/90 }}}, so
the 2nd car crosses first