Three people

Three people start doing a work at a same time. The first worked only 2 hours. The second ended 3 hours before the end. On an individual basis, it would take the first time to do the work 10 hours, second 12 hours and 15 hours third. How many hours did it take to work together under these conditions?

Correct result:

t =  7 h

Solution:

2/10 + (t-3)/12 + t/15 = 1

9t = 63

t = 7

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