
Dilution Dilemma
I am trying to find a formula to calculate dilution rates.
I know if I add 5 litres of a 20% solution to 15 litres of water I get a 5% solution because I have been told.
However if possible I would like the formula for the above example so I can calculate it for myself and workout for different final solution rates.
Can anyone help me please or point me in the right direction.

Dilution Factor = New Volume/Old Volume
New concentration = Old Concentration/Dilution Factor

Many thanks Pim.
But as my name suggests  I am not that quick.
With the numbers I already have  would you put them into those formulae please  so I can see what values come from where.

Hi
Let $\displaystyle \tau_1$ and $\displaystyle V_1$ be the dilution factor and the volume before dilution
The quantity of Idon'tknowwhat inside the water is $\displaystyle \tau_1\:V_1$
When you add $\displaystyle V_2$ litres of water the dilution factor becomes
$\displaystyle \tau = \frac{\tau_1\:V_1}{V_1+V_2}$
$\displaystyle \frac{0.20\:\cdot\:5}{5+15} = 0.05$

Thank you  that really has cleared up a problem I have been trying to work out for months.