Hi there, I am in grade 8 and have been given this question as part of an advanced placement program, I know that I can solve it using factoring or quadratic eqations but no matter what I do I do not seem to come up with the right answer.

Could someone please put me on the right track as to what I need to do to solve it?

Here is the problem. :eek:

You have a bottle of Deadly poison. You pour out 8 litres, and fill it with water. You pour out another 4 litres of the poison-water Mixture and fill it with more water. Now you have a mixture in which the poison concentration is 72%.

Find the capacity of the bottle.

Thanks, I look forward to your answers

Suppose capicity of bottle is x litres.

You take out 8 litres and fill with water.

Now you have concentration = (x -8)/x

You take out 4 litres of this

So really, you take out 4(x-8)/x litres of poison

Total poison taken out = 8 + 4(x-8)/x

Final concentration = 72%

100 - 72 = 28

8 + 4(x-8)/x = 28% of x = 28x/100

Simplify and Solve the quadratic: 8 + 4(x-8)/x = 28x/100

You will get 2 values of x, only one will work!

Answer : x = 40 litres