Assume that 2.5% ointment means that the remaining 97.5% is another substance mixed with it (petroleum). 2.5% is 2.5/100=5/200=1/40, so 97.5% is 1-1/40=39/40. 1/40 of 60g=60/40=3/2=1.5g. The quantity of the other substance is 60g-1.5g=58.5g. Add x grams of the other substance to make 60+x of the mixture or solution. So now we have 1.5g of hydrocortisone in 60+x g of mixture or solution and this must have a strength of 2%. Therefore, 1.5/(60+x)=2/100=1/50=0.02, and multiplying both sides by 60+x we get 1.5=(60+x)/50. Multiply through by 50: 75=60+x, so 15=x or x=15g. So 15g of petroleum need to be added to 60g of ointment to reduce the strength from 2.5% to 2%. (1.5/60=2.5%; 1.5/75=2%.)