Sanjay and Amit can do a piece of work in 8 days. Amit alone can do it in 12 days. In how many days will Sanjay do it alone?

Correct option is B

Given:

Sanjay and Amit can do a piece of work = 8 days
Amit alone can do it = 12 days.
Solution:
Sanjay and Amit's 1 day work =  $$\frac{1}{8 }$$​unit
Amit’s 1 day work = $$\frac{1}{12}$$​ unit
Then, for Sanjay’s one day work
Sanjay‘s one day work =  $$\frac{1}{8}-\frac{1}{12}=\frac{ (3-2)}{24}=\frac{1}{24}$$

Sanjay’s one day work =  $$\frac{1}{24}$$ unit ​
Therefore, Sanjay alone do the total work in 24 days.
Alternate Method:

Sanjay and Amit's efficiency = 3 unit
Amit’s efficiency = 2 unit
Now, for Sanjay’s efficiency = 3 – 2 = 1
Sanjay’s efficiency is 1 unit.
So, Sanjay can do 24 unit work in = $$\frac{24}{1}$$​ = 24 days