A can do a piece of work in 12 days and B in 24 days. If they work together, in how many days will they finish the work ?

Answer: Option B

Days	Eff.	Total work
A - 12	2
		24
B - 12	1
	3

A and B together can finish the work
$$\begin{array}{rl}& =\frac{24}{3}\\ & =8\text{ days}\end{array}$$