If 1 inch represents 10 miles on a map, then how many inches will represent 900 miles?

A simple way to find this using mathematical methods is to set up a proportion:

Then you would cross multiply getting:

900=10x (then divide by 10 on both sides to get the variable alone)

x=90

So 90 inches will represent 900 miles

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