Chlorine

Chlorine exists as two isotopes 35Cl and 37 Cl in the ratio 3:1
Which statement about peaks in the mass spectrum of cl2 is correct.
The answer is peaks at m/z=70, 72 and 74 in the ratio 9:6:1
Can someone explain why this is the answer
Thanks

A chlorine molecule is Cl-Cl.
A Cl atom can be either 35Cl or 37Cl.
That means that the possibilities for Cl2 are: 35Cl-35Cl (molecular mass = 70), 35Cl-37Cl (or the other way round) (mass 72), and 37Cl-37Cl (mass 74).
75% of Cl atoms are 35Cl, and 25% 37Cl (3:1 ratio).

Both atoms 35Cl: 0.75 x 0.75 = 9/16
35Cl-37Cl: 0.75 x 0.25 = 3/16
37Cl-35Cl: 0.25 x 0.75 = 3/16
37Cl-37Cl: 0.25 x 0.25 = 1/16

The 35-37 combinations add up to 6/16

And bosh you have the ratio 9:6:1