A can do a work in 15 days and B in 9 days. If they work on it together for 5 days, then what fraction of work is left?

Let the total work be the LCM of 15 and 9, which is 45 units.Efficiency of A = $\frac{45}{15}=3\text{ units/day}$ Efficiency of B = $\frac{45}{9}=5\text{ units/day}$ Combined efficiency of A and B = $3+5=8\text{ units/day}$ Work done by them together in 5 days:
$$\text{Work done}=8\times 5=40\text{ units}$$
Remaining work:
$$\text{Remaining work}=45-40=5\text{ units}$$
Fraction of work left:
$$\text{Fraction left}=\frac{\text{Remaining work}}{\text{Total work}}=\frac{5}{45}=\frac{1}{9}$$