Jerome's brother says with one cup of water for every one cup of milk, it is possible to keep the proportion of water and milk.
In order to determine if Jerome's brother is right, you first calculate the proportion between water and milk for the first pair of values of the table.
To calculate the proportion you divide both values, just as follow:
8/3 = 2.66
The previous result is the initial proportion between water and milk. You calculate if for other pair of values of water and milk tha proportion is the same. For instance, for the following values:
5/10 = 0.5
6/11 = 0.54
As you can notice, the proportion of water and milk is different.
Hence, Jerome's brother is wrong.