Suppose that apples are X yuan per kilogram and melon seeds are Y yuan per kilogram.

According to the meaning of the question:

2.5X + 1.5Y =30.5... 1

2X + 3Y =46...2

1 type +2 type

4.5X + 4.5Y = 76.5

So X+Y = 17.

So 1.5X+ 1.5Y = 25.5.

So I overcharged 0.5 yuan.