Darren Lee's answer to Eunice Chua's Secondary 1 Maths question.

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Darren Lee
Darren Lee's answer
7 answers (Tutor Details)
Since both Denny and John are moving together, we need to focus on the difference in speed instead of their individual speed. For John to catch up with Denny, the difference in speed (150 metres per minutes) is the one that is affecting how long he will take.
Therefore we take the 1800 metres and divide by the difference in speed, we get the time taken for John to catch up.