Explanation:
percentages are represented by hundredths in calculations.
2 1/2% = 2.5% = 0.025
2% = 0.02
we are mixing the 3 L 2.5% milk with x L of 0% milk.
and we are ending up with (3 + x) L of 2% milk.
so, we have
3×0.025 + x×0 = (3 + x)×0.02 = 3×0.02 + 0.02x
3×0.005 = 0.02x
0.015 = 0.02x
x = 0.015/0.02 = 0.75
so, 0.75 L have to be added to dilute the milk to 2%.