probability

P(at least one of the components A and Bfunctions normally)
= 1 - P(both A and B not function normally)
= 1 - (1 - 0.9)²
= 1 - 0.1²
= 0.99

P(at least 3 of the components C, D, E, F function normally)
= P(3 of the components C, D, E, F function normally) + P(all of C, D, E, Ffunction normally)
= C(4, 3) x 0.8³ x (1 - 0.8) + 0.8⁴
= 4 x 0.512 x 0.2 + 0.4096
= 0.8192

P(the electric circuit works)
= P(at least one of the components A and B functions normally)
.... x P(at least 3of the components C, D, E, F function normally)
= 0.99 x 0.8192
= 0.811008