Question 1039834
Let {{{ t }}} = time in hrs when Elena ( the faster one ) has 
traveled {{{ d }}} miles and Chris ( the slower one ) has traveled
{{{ d - 30 }}} miles.
Note that they start at the same time and
from the same place
------------------------
Elena's equation:
(1) {{{ d  = 14t }}}
Chris's equation:
(2) {{{ d - 30 = 10t }}}
---------------------
Substitute (1) into (2)
(2) {{{ 14t - 30 = 10t }}}
(2) {{{ 4t = 30 }}}
(2) {{{ t = 7.5 }}}
They will be 30 mi apart in 7.5 hrs
---------------------
check:
(1) {{{ d  = 14t }}}
(1) {{{ d  = 14*7.5 }}}
(1) {{{ d = 105 }}} mi
and
(2) {{{ d - 30 = 10t }}}
(2) {{{ d - 30 = 10*7.5  }}}
(2) {{{ d = 30 + 75 }}}
(2) {{{ d = 105 }}} mi
OK
( this tells you Elena's distance was 105 mi
and Chris's distance was 75 mi