Question 683698
Start a stopwatch when the express train leaves.
Now both will travel for the same amount 
of time, {{{ t }}} in hrs, until the express train 
overtakes the freight train.
Let {{{ d }}} = the distance the express train has to go
in km until they meet.
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Equation for freight train:
(1) {{{ d - 150 = 30t }}}
Equation for express train:
(2) {{{ d = 60t }}}
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Substitute (2) into (1)
(1) {{{ 60t - 150 = 30t }}}
(1) {{{ 30t = 150 }}}
(1) {{{ t = 5 }}}
The express train will overtake the freight train in 5 hrs
check answer:
(2) {{{ d = 60*5 }}}
(2) {{{ d = 300 }}}
and
(1) {{{ d - 150 = 30*5 }}}
(1) {{{ d = 150 + 150 }}}
(1) {{{ d = 300 }}}
OK