Math Destroyer Question

[sb6767]

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Can someone please help me understand how this Math Destroyer problem is solved?

A 120 minute film is started at normal speed wile a copy is started in reverse at twice the normal speed. After how many minutes will the two screens show the exact same frame simultaneously?

The answer is 40 minutes.

 

[Frank22]

So the number of minutes remaining for the film starting at 120 minutes is 120 - t minutes. The other film starts at 0 minutes remaining, and increases by 2t minutes. Our goal is to find where t is the same for both, so simply set the two equations equal to each other.

120 minutes - t minutes = 0 minutes + 2t minutes
= 120 min. - t min. = 2t min.
120 min. = 3t min.
40 minutes = t