Question

How many grams of 0.5% hydrocortisone cream must be mixed with 180 g of 2.5 % cream to make a 1 % w/w hydrocortisone cream? HINT: Use alligation method.

Answer 1

We need 540 grams of 0.5% hydrocortisone cream

Explanation:

Lets call x the amount of grams needed. This means that the combined cream will have 180+x grams. We want x such that

x * 0.005 + 180*0.025 = (180+x)*0.01

Hence

0.005x + 4.5 = 1,8+0.01x

Lets put the x in the same side and take common factor

4.5-1.8 = 0.01x-0.005x

2.7 = (0.01-0.005)x = 0.005x

As a result

x = 2.7*0.005 = 540

We need 540 grams.