Answer: Option B
:
B
Method 1: Using variables
Let the no of pipes be x.
Number of pipe days needed to fill tank A is $\frac{x}{2}+\frac{x}{4}$
This is double the amount taken to fill tank B....
The equations is,$\frac{\frac{x}{2}+\frac{x}{4}}{2}=(\frac{x}{4}+1)$
Thus, x=4.
Method 2:Using numbers (Assumption)
Assume answer to be option (b) - a middle answer option
Let A have a capacity of 300 litres
B will have a capacity 150 litres
Now 4 pipes fill A for half a day = effectively 2 pipes working for an entire day = 100 litres are filled by the first half
And 2 pipes fill A for another half = effectively 1 more pipe fills A for an entire day
This means that 3 pipes fill A completely= 300 litres
Thus, each pipe fills 100 litres
Let us verify it with B
Given that in the first day, 2 pipes fill B for half a day $\Rightarrow$ one pipe fills B for an entire day = 100 litres
Now 50 litres is left which Is filled by one pipe in half a day.
TOTAL = 150 litres for B
Thus, our assumption is correct