Well, here is the answer and the equation. Hope you understand it:
5+3=8
1/8 = 0.125
Move the decimal point 2 times to the left to get the percentages.
0.125 = 12.5%
concentrate: 12.5% x 3 = 37.5% or 100% - 62.5% = 37.5%
cold water: 12.5% x 5 = 62.5% or 100% - 37.5% = 62.5%
Get it now?