> if i do SELECT (1800/3600)*10 FROM RDB$DATABASE the result is 0.

The SQL standard defines the division of 2 integers to return an
integer. Thus 1800/3600 will return 0. 0 multiplied by anything = 0.

So Firebird is correct as per the SQL standard. Whether the SQL
standard makes sense is a question for another day.

> how do i get 5?
>

You need to make sure you are not dividing an integer by an integer.
This can be done by adding a decimal point if they are constants

eg:

1800.0 / 3600

or

1800 / 3600.0

If they are integer fields you are trying to divide, then you can use
cast.

cast(1800 as float) / 3600

Note that only one of the fields needs to be float or numeric to avoid
the integer division.

Adam