I will have a go at Q3 now, but then I have to go do something else for a while ...

"A pool can be filled with water by a large pipe within 6 hours. A smaller pipe will take 9 hours to fill the pool. How long will it take to fill the pool if the two pipes operate together?"

Hmmm .. let us imagine that the big pipe can pump 1000 litres an hour. I could say "x" an hour, but let's just try with some real numbers for a change.

So the pool would get 6,000 litres in 6 hours.

Now, the smaller pipe takes 9 hours, so it must be pumping 6,000/9 = 666 litres an hour.

TOGETHER (assuming they still work just as well) they would pump 1,000+666 = 1,666 litres an hour.

So it would take 6,000/1,666 = 3.6 hours

If this is homework, you should use proper algebra, something like:

Let x be the rate of flow of the large pipe.

The rate of flow of the small pipe is (6/9)x

The flow rate together is (1+6/9)x

Now, we know that a flow rate of "x" takes 6 hours, so a flow rate of (1+6/9)x should take 6 / (1+6/9) hours:

Time = 6 / (1+6/9) hours

Multiply top and bottom by 9 to simplify:

Time = 9*6 / 9*(1+6/9) = 9*6 / (9+6) = 54/15 = 3.6