Your ratio gives 3 private homes for every 8 (=3+5) houses.
This means as a fraction you have of the total amount of houses are privates homes.
If of the private homes are wooden, then at most, of them are brick. Multiply by to get the fraction of private homes that are brick (or the most that can be brick).
Add to this the fraction of apartments (since they are ALL brick) to get...
. This is the fraction of houses that are brick (at most).
To convert this to a ratio you get 80-77) = 77:3" alt="7780-77) = 77:3" /> which is the maximum ratio of brick houses to wooden houses.