# Thread: What does "None" mean here?

1. ## What does "None" mean here?

Hello. What does "None" mean of choice (d)?
Does it mean "it's impossible to calculate"? Thank you.

--------------------from Dare to Disappoint by Ozge Samanci
A swimming pool can be filled by Valve X in 6 hours.
When the swimming pool is full, Valve Y can drain half of the pool in 9 hours.
Valve Z can drain the entire pool in 10 hours.
If the swimming pool is empty, how many hours will it take
to fill the pool with all three valves open?

(a) 6 (b) 5 (c) 7 (d) None

2. ## Re: What does "None" mean here?

I never could do these maths problems.

I'm guessing that 'None' means that the water will drain away as fast as it's running in.

3. ## Re: What does "None" mean here?

It's not a very clear question, but I guess it means that none of the other answers is correct.

4. ## Re: What does "None" mean here?

It's badly worded. If they meant "None of the above", they should have written "None of the above". If they meant "0", they should have written "0".

5. ## Re: What does "None" mean here?

I figure it will take 90 hrs. Does anyone agree?

6. ## Re: What does "None" mean here?

Wow, did you actually solve this problem?

7. ## Re: What does "None" mean here?

Me too: 90 hours.

The rates that the valves fill the pool are as follows:

Valve X: 1/6 pools per hour
Valve Y: -1/18 pools per hour (9 hours to drain half a pool = 18 hours to drain one pool)
Valve Z: -1/10 pools per hour

Total flow rate: 1/6 - 1/18 - 1/10 = 1/90 pools per hour

So 90 hours per pool.

8. ## Re: What does "None" mean here?

I think "None" here means that is impossible to fill up the empty swimming pool with there valves opened.

9. ## Re: What does "None" mean here?

Originally Posted by Jolieshen
I think "None" here means that is impossible to fill up the empty swimming pool with there the valves opened.
Although "opened" is not wrong, most BrE speakers would use "open" there.

10. ## Re: What does "None" mean here?

Welcome to the forum, Jolieshen, but please read the forum rules and posting guidelines, one of which reads

You are welcome to answer questions posted in the Ask a Teacher forum as long as your suggestions, help, and advice reflect a good understanding of the English language. If you are not a teacher, you will need to state that clearly in your post.

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