Question 644568
With this type of problem, I like to think of 
one of them standing still and the other one
moving at the sum of their speeds
Let Sarah's speed  = {{{ s }}} mi/hr
John's speed = {{{ s + 15 }}} mi/hr
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The equation of motion is
{{{ 375 = ( s + s + 15 )*3 }}}
{{{ 375 = ( 2s + 15 )*3 }}}
{{{ 375 = 6s + 45 }}}
{{{ 6s = 330 }}}
{{{ s = 55 }}}
{{{ s + 15 = 70 }}}
Sarah is traveling at 55 mi/hr
John is traveling at 70 mi/hr
check answer:
Sarah's distance = {{{ 55*3 }}}
{{{ 55*3 = 165 }}} mi
John's distance = {{{ 70*3 }}}
{{{ 70*3 = 210 }}} mi
{{{ 165 + 210 = 375 }}} as it should
OK