# What does "None" mean here?

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#### frindle2

##### Member
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

##### Moderator
Staff member
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

##### No Longer With Us (RIP)
Staff member
It's not a very clear question, but I guess it means that none of the other answers is correct.

#### bubbha

##### Senior Member
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

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

#### frindle2

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

#### bubbha

##### Senior Member
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

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

#### emsr2d2

##### Moderator
Staff member
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

##### Moderator
Staff member
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

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

#### Tdol

##### No Longer With Us (RIP)
Staff member

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