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t is the time for which the first train has been travelling... since the second train left 2 hours later... it travels 2 hours less as compared to the first... hence t-2... additionally if you put t+2 into the eqn you will see that you get a -ve time... which isnt possible... of course you could always say t is the time taken by the second train... t+2 is then the time taken by the first train...
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