As part of a promotion a toy is put in each packet of crisps sold. There are eight different toys available. Each toy is equally likely to be found in any packet of crisps.
David buys four packets of crisps.
a. Find the probability that the four toys in these packets are all different.
b. Of the eight toys in the packets, his favourites are the yo-yo and the gyroscope. Find the probability that he finds at least one of his favourite toys in these four packets.
I'm stumped on b. Can someone explain how to do part b to me please?