Math Is Fun Forum

evene

Member

Registered: 2015-10-18

Posts: 272

Need help with this!

How many liters of water must be added to 7 liters of 20% acid solution to obtain a 10% acid solution?

I have this equation where a is the liters of water, but I got a negative number.

a+20%*7=10%*(a+7)

bobbym

bumpkin

From: Bumpkinland

Registered: 2009-04-12

Posts: 109,606

Re: Need help with this!

That does get a negative answer. I would add 7 liters of water.

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In mathematics, you don't understand things. You just get used to them.
If it ain't broke, fix it until it is.
Always satisfy the Prime Directive of getting the right answer above all else.

Bob

Administrator

Registered: 2010-06-20

Posts: 10,643

Re: Need help with this!

hi evene,

I did this with a different equation, but here's how to do it using 'a' litres added.

Before any water is added there are 4/5 x 7 litres of water = 28/5

Then add a to give (a + 28/5) water.

The acid is unchanged so it is 1/5 x 7 = 7/5 acid

To make a 10% mix  water:acid = 9:1 so

7/5 x 9 = a + 28/5

Bob

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Children are not defined by school ...........The Fonz
You cannot teach a man anything;  you can only help him find it within himself..........Galileo Galilei
Sometimes I deliberately make mistakes, just to test you!  …………….Bob