When i divide into ratio, i like to think of them as units:
so 24 will be divided into units, where the first thing gets 3 units and the second 5 units. In total those are 3+5=8 units.
so each unit will have 24/8=4
so the division into the two numbers is like this: the first one gets 3 units (3*4=12) and the second 5 (5*4=20)
so it's 12:20.