Question

person A and B can do a work in seven and half days and A alone can complete the work in twenty days how long it will take to person B to do it alone​

Answer 1

$\frac{1}{a} + \frac{1}{b} = \frac{1}{7} \\ \\ \frac{1}{a} = \frac{1}{20} \\ \frac{1}{20} + \frac{1}{b} = \frac{1}{7} \\ \frac{1}{b} = \frac{1}{7} - \frac{1}{20} \\ \frac{1}{b} = \frac{13}{140}$

The answer is 10.7 days