The efficiency of A, B and C are in the ratio 5 : 6 : 9. Working together, they can complete a work in 18 days. In how many days can B alone complete 25% of that work?

ratio of efficiency A:B:C=5:6:9

ratio of time taken A,B and C =$$\frac{90}{5}:\frac{90}{6}:\frac{90}{9}$$

=18:15:10

Now from question

$$\frac{x}{15}+\frac{x}{10}+\frac{x}{10}=\frac{1}{18}$$

$$\frac{x\left(5+6+9\right)}{90}=\frac{1}{18}$$

$$x=\frac{1}{4}$$

time taken by B to finsh the work=$$\frac{15}{x}=15\times\ 4=60\ days$$

time taken by B to finsh 25% the work=$$\frac{\left(60\times\ 25\right)}{100}=15\ days$$