Bunuel wrote:

If both conveyer belt A and conveyer belt B are used, they can fill a hopper with coal in 1 hour. How long will it take for conveyer belt A to fill the hopper without conveyer belt B?

(1) Conveyer belt A moves twice as much coal as conveyer belt B.

(2) Conveyer belt B would take 3 hours to fill the hopper without conveyer belt A

(1) Lets say belt B moves 'x' units of coal per hour, that means belt A can move '2x' units per hour. And since they are able to fill the hopper in 1 hour, that means the hopper contains = x + 2x = 3x units of coal.

Now hopper contains 3x units of coal and belt A can fill 2x units of coal per hour so time taken by belt A to fill the hopper alone will be = 3x/2x = 3/2 hours.

Sufficient.

(2) Belt B can fill hopper in 3 hours alone. So it fills at the rate of 1/3. And together since they are able to fill in 1 hour, that means belt B fills at the rate of 2/3 (because then only 1/3 + 2/3 = 1). Since belt A fills at the rate of 2/3 hopper per hour, the time it will take to fill the hopper alone = 3/2 hours.

Sufficient.

Hence

D answer