Question

When using a 1% cream and a 6% cream to produce a 4% cream, how many parts of each initial ingredient will be needed?

Answer 1

Here's visual way to calculate, which will let you do it mentally:

given cream concentrations
1% ----- 4% ----- 6%
----3%--------2%---- Difference

Then you take 2 parts of 1% and 3 parts of 6% to get 4%.