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

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%.