the usual way:
we can write two equations:
A=w*d, where A is total amount of job.
A=(w+n)*x, where x is the number of days, required for w+n women to do A.
w*d=(w+n)*x --> x=w*d/(w+n)
fast approach: It is obviously that more women (n increases) lead to fewer days to perform the same job. So, if we put a huge n, we have to obtain ~0. All options, except A, increase when n increases.
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