A is 1.5 times as efficient as B. A & B together complete the work in 12 days. In how many days, A alone can complete the work?

Let's assume the total work is 60 units.

A & B together complete the work in 12 days.
The efficiency of A & B together = $$\frac{60}{12}$$

= 5 units/day

A is 1.5 times as efficient as B.

the efficiency of A = 1.5 of the efficiency of B

the efficiency of A : efficiency of B = 3:2

efficiency of A = 3 units/day

efficiency of B = 2 units/day

Time taken by A alone to complete the work = $$\frac{60}{3}$$

= 20 days