Girls : 2 stickers each
Boys : 3 stickers each

Number of girls = 5 x number of boys


Method : grouping


Put 1 boy and 5 girls into a group.

No matter how many groups there are, the total number of girls is always 5 times the number of boys

Number of stickers in a group
= 1 x 3 + 5 x 2
= 3 + 10
= 13

Number of groups such that there's a total 390 stickers

= 390 ÷ 13
= 30

So there are 30 groups.

Thanks

In each group , the difference in number of stickers boys have and girls have

= 10 - 3
= 7

Girls have 7 more stickers than boys in each group

Since there are 30 groups,

7 x 30 = 210

All the girls have 210 more stickers than all the boys

Welcome. See my latest comment for the answer.