Ratio and stratified sample

Question

The ratio of children to adult visitors is 2:3. The ratio of adults to senior citizen visitors is 4:5. A stratified sample of 70 is to be taken from the visitors to the theatre. How many senior citizens should be in the sample?

Answer 1

c : a :: 2 : 3

a : s :: 4 : 5

or

c : a :: 8 : 12

a : s :: 12 : 15

ratio of seniors to total

15 : 35

15 : 35 :: x : 70

x = 30