Store B 54 yuan = 1 box +3 pieces =42 yuan+12 RMB.

Comparing the prices of Store A and Store B, we only need to see if 12 yuan =3 pieces, that is, 1 piece =4 yuan is worth it.

We take 42÷4= 10.5,

Then, when the number of 1 boxes is not less than 1 1, the unit price of each box is less than 4 yuan, that is, 1 box =4 yuan is not worth it, and then Store A is cheaper;

When 1 box number is less than 1 1, the unit price per box is greater than that in 4 yuan, that is, 1 box =4 yuan value, so it is cheaper to buy it in the second store.