needhelp1700

It’s just the multiplication rule.
There is a .8 probability that each bond will NOT default. Therefore, for all 5 bonds to NOT default it’s .8 x .8 x .8 x .8 x .8
For 1 bond to default, it’s the probability of 1 bond defaulting (.2) multiplied by the probability of the other 4 bonds NOT defaulting (.8)
Therefore it’s .2 x .8 x .8 x .8 x .8