What does this rebus riddle means ?

A large water tank has two inlet pipes (a large one and a small one) and one outlet pipe. It takes 3 hours to fill the tank with the large inlet pipe. On the other hand, it takes 6 hours to fill the tank with the small inlet pipe. The outlet pipe allows the full tank to be emptied in 9 hours.

What fraction of the tank (initially empty) will be filled in 0.64 hours if all three pipes are in operation? Give your answer to two decimal places (e.g., 0.25, 0.5, or 0.75).

Rearrange the letters in "new door" to make one word.

Can you find out the missing number?

? 4 5 3 5

9 5 7 10 3

You order chicken wings at KFC in the boxes of 6, 9 and 20. What is the largest number of wings that you cannot obtain by buying in any combination of the boxes?

A word I know,
Six letters contain,
Subtract just one,
And twelve is what remains.

I am a seven-letter word but if you remove four of my letters I will have one left. What word am I?

Find the value of "?" in the ball picture Riddle below:

The person who makes it has no need for it. He who purchases it does not use it. The person who does use it does not know they are. What is it?

A word I know has six letters it contains, remove one letter and 12 remain. What is it?