Question 555998
You need 2 equations: 1 for Elena and 1 for Javier
Let {{{ s }}} = Javier's speed in km/hr
Let {{{ t }}} = the time for both
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Javier's eqution:
(1) {{{ 57 = s*t }}}
Elena's equation:
(2) {{{ 75 = ( s + 6 )*t }}}
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This is 2 equations and 2 unknowns, so it's solvable
(1) {{{ t = 57/s }}}
substitute (1) into (2)
(2) {{{ 75 = ( s + 6 )*(57/s) }}}
(2) {{{ 75 = 57 + 342/s }}}
(2) {{{ 75s = 57s + 342 }}}
(2) {{{ 18s = 342 }}}
(2) {{{ s = 19 }}}
{{{ s + 6 = 19 + 6 }}}
{{{ s + 6 = 25 }}}
Javier's speed is 19 km/hr
Elena's speed is 25 km/hr
check answer:
(1) {{{ t = 57/s }}}
(1) {{{ t = 57/19 }}}
(1) {{{ t = 3 }}} hrs 
and
(2) {{{ 75 = ( s + 6 )*t }}}
(2) {{{ 75 = 25t }}}
(2) {{{ t = 75/25 }}}
(2) {{{ t = 3 }}} hrs
OK