In what proportion should 5% and 1% hydrocortisone ointments be mixed to prepare a 2.5% ointment?

Question

Applying: Altering Strength, Stock Solutions, and Alligation Calculations

Answer 1

Alligation:

Have.........Need..........Parts

5%...............................1.5

..................2.5%................

1%..............................2.5

Use 1.5 parts of the 5% and 2.5 parts of the 1%

In other words, they should be mixed in a proportion of 1 : 0.6 (5% : 1%)

let x = parts of 5%

1 - x = parts of 1%

(x)(5%) + (1-x)(1%) = (1)(2.5%)

5x + 1 - x = 2.5

4x = 1.5

x = 0.375 parts of 5%

1-x = 0.625 parts of 1%

The proportion is 1 : 0.6 (5% : 1%)