A does 30% of a work in 30 days. He then calls in B and together, they finish the remaining work in 20 days. How long would B alone take to do the whole work?

A does 30% of the work in 30 days, so A's daily work rate is 1% per day, meaning A alone would take 100 days. The remaining work is 70%, and A and B finish this in 20 days, so their combined rate is 70/20 = 3.5% per day. Subtracting A's 1% daily rate, B's daily rate is 2.5% per day. By inverting this rate, B alone takes 40 days to do the whole work.

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