Question

a box contains 25 computer diskettes, 5 of which are double density and the remiander are high density. A second box contains 15 diskettes, 10 double density and 5 high density. A box is selected at random and then two diskettes are chosen randomly from the selected box. What is the probability that both diskettes are high density?

Answer 1

The probabality of selecting a HD is

(20+5)/(25+15) * (24)/(39)

There are longer and more complicated of working this out.

Answer 2

can you explain?