Question 1158063
The head start of the 1st train is:
{{{ d[1] = 60*(1/2) }}}
{{{ d[1] = 30 }}} km
Let {{{ d }}} = the distance the 2nd train travels
until it catches the 1st train
Let {{{ t }}} = time in hrs for the 2nd train to 
catch the 1st train
Start a stopwatch hen the 2nd train leaves
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Equation for the 1st train:
(1) {{{ d - d[1] = 60t }}}
Equation for the 2nd train:
(2) {{{ d = 75t }}}
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(1) {{{ d - 30 = 60t }}}
Plug (2) into (1)
(1) {{{ 75t - 30 = 60t }}}
(1) {{{ 15t = 30 }}}
(1) {{{ t = 2 }}}
It takes 2 hrs for the 2nd train to catch the 1st train
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check:
(2) {{{ d = 75t }}}
(2) {{{ d = 75*2 }}}
(2) {{{ d = 150 }}} km
and
(1) {{{ d - d[1] = 60t }}}
(1) {{{ d - 30 = 60*2 }}}
(1) {{{ d = 30 + 120 }}}
(1) {{{ d = 150 }}} km
OK