A question has come up on a mock exam and i'm not sure if i have done it correctly so please can you tell me if i have done it wrong or if it is right?

Peter and Margaret's ages are in the ratio 4:5. If Peter is 16 years old, how old is Margaret?

Peter = 4/9

so 1/9 is 16 divided by 4 = 4

4 x 5 = 20

Margaret is 20.

Thank you in advance