I think the answer is 4/25 simplified to the lowest amount
Divide the number of flowers by 3 and that is how many rows there are
12 / 3 = 4
18 / 3 = 6
21 / 3 = 7
30 / 3 = 10
4c6d3 x 3c4d
4 x 3=12
12c
you add the powers so 6+4=10
12c10
d3 x d=d3
12c10d3
that is the answer i think