How do I update the value of one field according to another?

[sajaddp]

Hello
I have two tables A and B.
Table A has three columns called b_id, hour, and amount.
Table B also has a user_id and salary column.

The relationship between the two tables is also well defined.

When the user creates an A, I want the amount to be filled in automatically according to the following relation:
amount = hour * b.salary

How do I do this?

[anditsung]

using observer or event should solve this