Question 1184425
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Here are a couple of other ways to solve this and other similar problems.<br>
(1) Alexis takes twice as long to do the job as Mark, so she does half as much work in the same amount of time.  So when working together the two of them are like 3/2 workers. That means together it will take them 2/3 as long to do the job.
2/3 of 120 minutes is 80 minutes.<br>
(2) Consider the least common multiple of the two given times, which is 240 minutes.  In 240 minutes Mark could do the job 2 times and Alexis could do it once; so together in 240 minutes they could do the job 3 times.  And that means they could do the job together 1 time in 240/3=80 minutes.<br>