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1. A completes $$\frac{7}{{10}}$$ of the work 15 days. Then he completes the remaining work the help of B in 4 days. The time required for A and B together to complete the entire work is = ?

1.

A completes $$\frac{7}{{10}}$$ of the work 15 days. Then he completes the remaining work the help of B in 4 days. The time required for A and B together to complete the entire work is = ?