If A, B and C can do a work in 90, 30 & 45 days respectively. If they work together, in how many days will they complete the work?

A' s one days work  = 1/90

B' s one days work  = 1/30

C' s one days work  = 1/45

$A+B+C one days work=\frac{1}{90}+\frac{1}{30}+\frac{1}{45}$

$=\frac{1+3+2}{90}=\frac{6}{90}=\frac{1}{15}$

In 15 days they can complete the work