2. A 1, A2 and A3 are all disconnected at the same time, and 0.4x0.5x0.2=0.04.

3. When the line works normally, B 1 and B2 must be connected at the same time, and at least one of A 1, A2 and A3 can be connected.

So 0.7x0.9x (1-0.4x0.5x0.2) = 0.6048.