[求助]胡老师，请教一个问题～～～！！ 胡老师

liondaisy

Of all the bonds currently rated B, 20% will default over an investor's horizon. Th variance of the number of defaults in a randomly selected 40-bond protfolio over the investor's horizon?

The answer is : 0.2*0.8*40=6.4

But why?