Question

a can do a piece of work in 8 days, which b alone can do in 10 days. in how many days both working together can do it?

Answer 1

Together they can complete the work in
$\frac{8 \times 10}{8 + 10} = \frac{80}{18} = \frac{40}{9} =4 \frac{4}{9} \: \: days$