In a society there are 30 men and 20 women members. What is the probability that the person selected for
the post of the president is a woman?

This is because there are 20 women out of 50 people total (30+20 = 50).

So 20/50 = 2/5

This converts to the decimal form 0.4 and the percentage form 40%

if you are saying purchasing 3 social media ads and one newspaper ad counts as one purchase then there are:

6 purchases

if you are asking how many of each then:

18 social media ads

6 newspaper ads