a)
let x = total amount of sticker
1/6 x + 336 = 3/4 x
336 = 3/4x - 1/6x
336=7/12x
Hence, X=576
To answer (a), simply,
576/4=144.
b)
Using guess and check,
let a = friends that received 6 stickers
let b = friends that received 12 stickers
6a + 12b = 144
By using guess and check,
a = 16
b = 4
so to answer (b), the answer is 4.

If u need a non algebraic way..
We know that 1/6 and 336 was spent, resulting in 1/4 left. And we want to find out 336 is what fraction of her stickers.
1 - 1/6 - 1/4 = 7/12
So 336 is 7/12 of her total stickers.
1/4 number of stickers would mean 336/(7/12) * (1/4) = 144

So now 20 of her friends. If we were to give each of them 6 stickers. 20*6 = 120
144-120 = 24.
Now each friends have 6 stickers. For friends to have 12 stickers we just need to give each of them 6 more. 24/ 6 = 4