Question 1041719
You can imagine that one of the trucks is standing still 
and the other is moving at the sum of their speeds
Let {{{ s }}} = the speed in mi/hr of the 2nd truck
{{{ s - 15 }}} = the speed in mi/hr of the 1st truck
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{{{ 540 = ( s + s - 15 )*4 }}}
{{{ 540 = ( 2s - 15 )*4 }}}
{{{ 540 = 8s - 60 }}}
{{{ 8s = 600 }}}
{{{ s = 75 }}}
and 
{{{ s - 15 = 75 - 15 }}}
{{{ s - 15 = 60 }}}
The faster truck's speed was 75 mi/hr
The slower truck's speed was 60 mi/hr
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check:
{{{ d[1] = 75*4 }}}
{{{ d[2] = 60*4 }}}
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{{{ d[1] = 300 }}} mi
and
{{{ d[2] = 240 }}} mi
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{{{ d[1] + d[2] = 540 }}}
{{{ 300 + 240 = 540 }}}
{{{ 540 = 540 }}}
OK