Answer:

Step-by-step explanation:

If A can do this work in 12 days, he/she can do  112  of it in one day.

If B can do this work in 15 days, he/she can do  115  of it in one day.

If C can do this work in 20 days, he/she can do  120  of it in one day.

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If they can do these fractions of this work on one day, adding these numbers up, we can see how much of this work is done in one day by the three people working at the same TIME:  112+115+120=560+460+360=1260=15 . So the three people can do  15  of this work on one day. C left after two days, so before he left,  25  of this work were done. A does  112  of it every day, and B does  115  of it every day, so together, they do  112+115=560+460=960=320  of it every day. Since  25  of this work were done when C left, when only A and B started working on the PROJECT, there were  35=1220 of this work left. If  320  of it is done every day, it takes four days to complete the REST of this work. The remaining work will be completed by A and B in 4 days.