Relative frequency of plastic buttons with four holes is 28/80 = 0.35
Relative frequency of plastic buttons with two holes is 15/80 = 0.1875
Relative frequency of wooden buttons with two holes is 34/80 = 0.425
Relative frequency of wooden buttons with four holes is 3/80 = 0.0375
0.35 + 0.1875 + 0.425 + 0.0375 = 1