A. B and C can do a piece of work in 10, 15 and 30 days, respectively. If B and C both assist A on every third day, then in how many days will 40% of the work be completed?

Given, 

A. B and C can do a piece of work in 10, 15 and 30 days, respectively.

Let the total work be 30 units (LCM of 10,15 and 30) 

As we know, 

Efficiency or one day work = $$\frac{Total\ work}{time\ taken}$$

Efficiency of A = 3

Efficiency of B = 2

Efficiency of C = 1

Condition given : B and C assist A on every third day

i.e; On first and second day A alone do work

Work done in 3 days = 3 + 3 + 6 = 12 units

40% of the total work finished in = $$\frac{\left(\frac{40}{100}\times\ 30\right)}{12\ }=\ 3\ days$$