Why 216÷12

Every 3 girls have as many sticker as 2 boys.
Common multiple of 2 and 3 = 6

We group 3 girls and 2 boys in 1 group.
3 girls → 6 stickers
2 boys → 6 sticker
Total = 12 stickers

1 group → 12 stickers
? group → 216 sticker
= 216 ÷ 12 = 18 groups.

Each group have 3 girls
Total girls = 18 × 3 = 54

Each group have 2 boys
Total boys = 18 × 2 = 36

Anyway, questions have some error or lack of information.

Normally, question will give "They are twice as many boys as girl"...

Thanks for the explanation.

Well, this seems based on the assumption that the total number of sticker is same for B & G

But can't find this information in the Qn ?

Yes assumed they received same number of stickers, but question ask how many more boy than girls? Therefore need more information.