Calling All Math Mental Giants

[SJW]

This is probably the wrong section and probably the wrong web site but knowing u guys you are the only ones I could think of to answer my question.
My had a math question and he got it wrong. We dont know the answer and we cant work it out, sohere it is.


How much fuel do you need to add to 1 litre of 1:20 mix to make up to 1:30?

My family is over for lunch and no-one can work it out. Well we get plenty on answers but dont know what one is correct.

Steve

 

[SJW]

Sorry it should be on OFF TOPIC!


Moderator!

 

[Adamt]

1:20 mix means 1 part fuel to 20 parts water (If I interpret that correctly...). If you want to make it 1 part fuel to 30 parts water, you wouldnt add any fuel, you would add water....

 

[Pollux]

I read it as mixing fuel with say 2-stroke oil...

being the nerd I am, I used a spreadsheet.

 

[gruntus]

I think he means one part oil to 20 parts fuel....maybe....maybe not.....doohhh....too slow

1:20 mix means 1 part fuel to 20 parts water (If I interpret that correctly...). If you want to make it 1 part fuel to 30 parts water, you wouldnt add any fuel, you would add water....

 

[Adamt]

This is probably the wrong section and probably the wrong web site but knowing u guys you are the only ones I could think of to answer my question.
My had a math question and he got it wrong. We dont know the answer and we cant work it out, sohere it is.


How much fuel do you need to add to 1 litre of 1:20 mix to make up to 1:30?

My family is over for lunch and no-one can work it out. Well we get plenty on answers but dont know what one is correct.

Steve

Sorry it should be on OFF TOPIC!


Moderator!

I think you knew that you were putting it in somewhere where it would be on the latest threads list

Pollux wins.

 

[SJW]

Thanks Pollux that looks good. I will have a closer look and see if I can make sense of it.

 

[Wrenny]

Add half as much again.

For ever 1 oil, you had 20 fuel. Now you want 30 fuel, so you add another 10 (ie half a litre in your case)

 

[Stuster]

For a rough and ready calculation, you can say that 1/20th of the mix is oil (really 1/21st) which means there's 50ml oil. So if you want 30:1 mix you can multiply that 50ml by 30, meaning 1.5L. So you need to add 0.5L oil. Near enough for most purposes I'd say.

 

[Tony]

It says how much FUEL do you add.

The way i read it it is a 1 part oil, 20 parts fuel. In the question the oil stays the same and the fuel increases making it 30 parts of fuel to the origional 1 part of oil.

take the origional 1000ml volume, divide by the 20 parts, multiply the result by the 30 parts you require and subtract the origional 1000mls in question.

This gives you 500mls of fuel to add.

1000mls/20parts = 50
50 x 30parts = 1500mls
1500mls - origional 1000mls = 500 mls

easy

 

[Adamt]

Nope Tony... 1 part + 20 parts makes 21 parts in the total, so you must divide by 21 to get the volume of the 1 part.

 

[SJW]

Thats the way I was thinking Tony but with 1:20 or 1:30 u end up with 21 and 31 parts.

 

[SJW]

I read it as mixing fuel with say 2-stroke oil...

being the nerd I am, I used a spreadsheet.

Thats the one Pollux, I suspect the trick in the question was that extra part of oil making it effectivley 21 parts. 476mls is the correct answer. We all kept coming up with 500mls.

Steve

 

[Darren]

If it was 1/30 you are after then 500mls/litre is correct.

Do you have anyway to precisely measure 476ml of petrol?

1/30 or 1/31 isnt going to make one iotta of difference in a lawnmower

cheers

Darren

 

[mwd]

1/30 or 1/31 isnt going to make one iotta of difference in a lawnmower

cheers

Darren

Unless it's a Honda :lol: Smooowkin

Sorry couldn't resist.

 

[Pollux]

Darren, I'm guessing this is a younger relative's maths homework or similar.

 

[katzke]

Nope Tony... 1 part + 20 parts makes 21 parts in the total, so you must divide by 21 to get the volume of the 1 part.

Well the one part is still oil. So if you have 20 parts fuel and want 30 you do add half as much again.

Now if you have a left over amount and want to thin it then you may want to take into account the part that is not fuel. But we are splitting hairs here as no one can accurately measure form an oil bottle anyway. I mean who besides this cheep Yankee rinses out the single use pre measured oil bottles anyway. Yes I know they cost more but some times that is all I can find or I get them at yard sales real cheep.

 

[jonocarroll]

I'll just verify the question: You want to add some volume of fuel to 1L of a 1:20 oil:fuel solution to obtain a 1:30 solution.

- You start with 1L of 1:20 solution (20/21 of which is fuel).
- You add to this some volume of fuel to obtain 30/31 fuel.

The equations you need to solve are;

(20/21)*1L + FuelToAdd = (30/31)*TotalVolume [[ the total amount of fuel ]]
FuelToAdd + 1L = TotalVolume [[ the total volumes need to add up ]]

Various ways to do this, but Mathematica is the easiest. Last line is the output.



So, Pollux is correct. You would need to add 0.47619L of fuel.

The more common question (slightly trickier) is "how much pure fuel do you need to add to a 1:20 solution to make exactly 1L of 1:30 solution?"



in which case you would need 0.677L of 1:20 solution, and add 0.323L of pure fuel.