the corrent answer is 5/6 .

since both the guns are fired at the same time and both the guns do now hve any connection

so probability of 1st gun firing a round is 3/6.

and probability of 2nd gun firing = 2/6.

now we will add then wince they are mutually exclusive.

2/6 + 3/6 = 5/6

2/6*3/6 + 4/6*3/6 + 2/6*3/6 = 2/3 is correct.

1 - 4/6*3/6 = 2/3 is a better answer.