probability math word problem

Question

A particular system is composed of four machines: A, B, C, and D. The entire system will work as long as any one (or more than one) of the four machines works. (With back up).

P(A works) = 0.60

P(B works) = 0.55

P(C works) = 0.65

P(D works) = 0.62

Rounded to the fourth digit beyond the decimal point, P(The entire system works) =

Answer 1

This is 1 - the probability that none of them work = 1 - (1 - .6) (1 - .55) (1 - .65) (1 - .62) = .97606, which rounds to .9761