Help!!!Maths(probability)

a) If two dice are thrown, find the probability of rolling a sum less than 4.

(b) A pocket contains 12 balls, of which 5 are red and 7 are yellow. A boy takes 3 balls at random. Find the probability that the boy will get

(i) all 3 red balls,
(ii) at least 1 red ball.

c) 6 boys and 6 girls are arranged to be seated in a row at random.
(i) Find the number of possible ways that boys and girls are sitting alternatively

(ii) Suppose we have to re-arrange the seats into 2 rows, so that each row consists of 6 seats as shown below.

OOOOOO
OOOOOO

Suppose that boys and girls are still required to be seated alternatively and each boy sitting in the first row is required to have a girl sitting right behind him, and each boy sitting in the second row are required to have a girl sitting right in front of him. If we are still to arrange the seats at random, what is the probability that we have a correct arrangement?