You can put this solution on YOUR website! Assume the resulting mixture 4% cream is 100 units quantity of mixture.
x is quantity of 2% cream
y is quantity of 5% cream
(2x+5y)/100=4 and x+y=100 correspondingly for concentration and for sum of parts.
2x+5y=400
and
x+y=100
The numbers allow for fairly easy use of Elimination Method to start.
2x+5y=400
and
2x+2y=200
-
-
3y=200
y=66.7 approximately, but theoretically units.
Meaning, parts.
That would be for 100 units of finished 4% cream mixture.
Simpler meaning is, 1 part of 2% cream and 2 parts of the 5% cream.