Question

when using a 2 % cream and a 5% cream to produce a 4% cream how many part of each initiial ingredient are needed?

Answer 1

I saw a post by Mathmate that works these easier than an algebra solution.

............2%.......4%.........5%
difference........2.......1
So you take 1 part of 2% and 2 parts of 5% to make 3 parts of 4%.
As an example, if you wish to prepare 300 grams, take 100 g of the 2% and add to 200 g of the 5% to prepare 300 g of the 4%.