Probability

Cannon A hits: 0.3

Cannon A doesn't hit: 0.7

Cannon B hits: 0.4

Cannon B doesn't hit: 0.6

Cannon C hits: 0.5

Cannon C doesn't hit: 0.5

The probability that none of the cannons will hit the target after one round:

(A doesn't hit) x (B doesn't hit) x (C doesn't hit) = 0.7 x 0.6 x 0.5 = 0.21

The probability that at least one cannon will hit the target after one round:

1 - (A, B, and C all don't hit) = 1 - (none of the cannons hits) = 1 - 0.21 = 0.79

The probability that at least two cannons will hit the target after one round:

(A doesn't hit, B hits, C hits) + (A hits, B doesn't hit, C hits) + (A hits, B hits, C doesn't hit) + (A hits, B hits, C hits)

= (A doesn't hit) x (B hits) x (C hits) + (A hits) x (B doesn't hit) x (C hits) + (A hits) x (B hits) x (C doesn't hit) + (A hits) x (B hits) x (C hits)

= 0.7 x 0.4 x 0.5 + 0.3 x 0.6 x 0.5 + 0.3 x 0.4 x 0.5 + 0.3 x 0.4 x 0.5

= 0.35

The probability that all three cannons will hit the target after one round:

(A hits) x (B hits) x (C hits) = 0.3 x 0.4 x 0.5 = 0.06