Question 1112890
Let {{{ s }}} = the speed of the faster car in mi/hr
{{{ s - 21 }}} = the speed of the slower car
---------------------------------------------
You can think of this as one of the cars travelling
at the sum of their speeds and the other one 
standing still
{{{ 692 = ( s + s - 21 )*4 }}}
{{{ 4*( 2s - 21 ) = 692 }}}
{{{ 8s - 84 = 692 }}}
{{{ 8s = 776 }}}
{{{ s = 97 }}}
and
{{{ s - 21 = 76 }}}
--------------------
The faster train's rate is 97 mi/hr
The slower train's rate is 76 mi/hr
-----------------------------------
check:
{{{ d[1] = 97*4 }}}
{{{ d[1] = 388 }}} mi
and
{{{ d[2] = 76*4 }}}
{{{ d[2] = 304 }}}
and
{{{ d[1] + d[2] = 388 + 304 }}}
{{{ d[1] + d[2] = 692 }}}
OK