Question 853179
Think of one of them as standing still and
the other one moving at the sum of their speeds
Let {{{ t }}} = the time in hours when they will be 
{{{ 66 }}} miles apart
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{{{ 66 = ( 10 + 12 )*t }}}
{{{ t = 66/22 }}}
{{{ t = 3 }}}
They will be 66 miles apart in 3 hrs
check:
{{{ d[1] = 10t }}}
{{{ d[1] = 10*3 }}}
{{{ d[1] = 30 }}}
and
{{{ d[2] = 12*3 }}}
{{{ d[2] = 36 }}}
and
{{{ d[1] + d[2] = 30 + 36 }}}
{{{ d[1] + d[2] = 66 }}}
OK