How to count Collective failure rate for chips?
The collective failure rate is one or both may happen =
A + (1 - A) x B = A + B - A x B

The 0.81 is from A x B (=0.9x0.9).

I don't use statistics often, but for 3 elements, doing the above recursively, the combined rate =
A + B + C - AxB - AxC - BxC + AxBxC