Auto-calculate Values

I am thinking of how to write the code for this:

There are 3 textboxes, each labeled:

TOTAL
rd_PROPERTY_VALUE
LTV

Both “TOTAL” and “rd_PROPERTY_VALUE” allow you to enter any number (e.g. 10000, 2500, 12345, 100.10, etc.). So whenever I enter numbers in both of them, the 3rd read-only textbox will automatically divide TOTAL by rd_PROPERTY_VALUE then times it by 100 and show the calculated value in the “LTV” field.

For instance it should mathematically appear like this:
(TOTAL / rd_PROPERTY_VALUE) * 100 = LTV

Can anyone give me any references to this?