How to work out ratios?

Hi
How do you work out correct ratios?
example
19 children in - 4 members of staff
9 under 3yrs
10 over 3yrs

So for under 3yrs need 2 members of staff for 8 children..what happens to the 1 left?
10 over 3yrs needs 2 members of staff and they can include the 1 left over on the under 3yrs?

Its the one remaining under 3 yrs that gets me. How can they be included in a staff ratio that then is worked out for the over 3yrs...sorry confusing myself!
Does an under 3 yrs take two places of an over 3yrs?

Think someone on here had a point score system but I cant find it.