TANCET Math Work Time Question

Here is a work time question. You could expect a question on this topic. A topic that gets usually tested in the test.
Question
A works twice as fast as B does and together they complete a task in 8 days, how long will A lone take to complete the task?
1. 16 days
2. 4 days
3. 32 days
4. 12 days
5. 24 days

Correct Answer
Choice (4). 12 days.

Explanatory Answer
A works twice as fast as B does.
So, if B does 'x' units of work in a day, then A does 2x units of work in a day or in a day they do 3x units of work.
Together, they complete the task in 8 days.
i.e., in a day together they complete 1/8th of the total task.
3x = 1/8th of the task.
So, x = 1/24th of the task.
We know A does 2x units of work a day = 2 * (1/24) = 1/12th of the work in a day.
So, A will take 12 days to complete the task.