A is thrice good a workman as B and therefore is able to finish a job in 40 days less than B. Working together they can do it in:

Given,
A is thrice good workman as B. Means,
A = 3B
Let B can finish work in X days, then A will finish same work in (X - 40) days alone
Now,
BX = 3B × (X - 40)
X = 60 days
B can finish work in 60 days, then A can finish the work in 20 days.
One day work of B = $$\frac{1}{{60}}$$
One day work of A = $$\frac{1}{{20}}$$
One day work of (A+B) =
$$\frac{1}{{60}} + \frac{1}{{20}} \Rightarrow \frac{{1 + 3}}{{60}} \Rightarrow \frac{1}{{15}}$$
So, they can finish work together in 15 days