To help you think about what's happening, you might like to make up an amount of people in the theatre (I would recommend 35 as a good number) and then use that number to calculate how many end up buying ice cream. Does that give you a better idea of what's happening?
if 3/5 of the people in a theatre buy a snack during interval, and of those who buy a snack 5/7 buy ice cream, what fraction of people in the theatre buy ice cream?
3/5 * 5/7 = 3/7 = snack, icecream
3/5 * 2/7 = 6/35 = snack, no icecream
So 3/7 of the people in the theatre buy icecream as a snack, as 5/7 of 3/5 is 3/7.