Hey there :)
Let p represent the number of bags to be bought
We need to collect 30 stickers and each bag has 3 stickers
3p = 30
↑ ↑
Number of stickers in a bad Number of stickers to be collected
= 
p = 10
We need to buy 10 bags of crisps to have 30 stickers
Let p represent the number of bags to be bought
We need to collect 30 stickers and each bag has 3 stickers
3p = 30
↑ ↑
Number of stickers in a bad Number of stickers to be collected
p = 10
We need to buy 10 bags of crisps to have 30 stickers