Question 1147058
Since they are moving apart, their speeds will add.
You can think of one of them standing still and the 
other one moving at the sum of their speeds
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A.
Let {{{ d }}} = their distance apart after {{{ 2.5 }}} hrs
{{{ d = ( 10 + 5 )*2.5 }}}
{{{ d = 15*2.5 }}}
{{{ d = 37.5 }}} mi
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B.
Let {{{ t }}} = time in hrs for them to be {{{ 25 }}} mi apart
{{{ 25 = ( 10 + 5 )*t }}}
{{{ 25 = 15t }}}
{{{ t = 1.667 }}} hrs 
1 hr 40 min
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check answers:
A.
Simon's equation:
{{{ d[1] = 5*2.5 }}}
{{{ d[1] = 12.5 }}} mi
and
Jane's equation:
{{{ d[2] = 10*2.5 }}}
{{{ d[2] = 25 }}} mi
and
{{{ d[1] + d[2] = 12.5 + 25 }}}
{{{ d = 37.5 }}}
OK
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B.
Simon's equation:
{{{ d[1] = 5t }}}
and
Jane's equation:
{{{ d[2] = 10t }}}
{{{ d[1] + d[2] = 5t + 10t }}}
{{{ 25 = 15t }}}
{{{ t = 1.667 }}} hrs
OK