What does "None" mean here?

[frindle2]

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

 

[Rover_KE]

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.

 

[Tdol]

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

 

[bubbha]

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".

 

[Raymott]

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

 

[frindle2]

Wow, did you actually solve this problem? :up:

 

[bubbha]

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.

 

[Jolieshen]

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

 

[emsr2d2]

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

Although "opened" is not wrong, most BrE speakers would use "open" there.

 

[Rover_KE]

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.

 

[Jolieshen]

I answer this question last week, but why I can't see it today??

 

[Tdol]

Try refreshing/reloading the page. I can see your answer a few post earlier in the thread.