A store buys 5 pieces for 1 piece, which is equivalent to buying 6 pieces and only giving 5 pieces.

So 48÷6=8

5*8=40

40* 10=400 yuan

A 10% discount in store B is equivalent to a pen that only needs 9 yuan.

So 48*9=432 yuan.

Because the total amount of money in store C is 48 * 10 = 480 < 500.

So buy two more,

At this time, 50* 10*0.8=400 yuan.

So I chose Store C and spent 400 yuan money to buy 50 pieces.