prototyping tut - interest rates

Prototyping tutorial

A finance company wants a program - to be used by customers - which calculates the interest on an amount, given the interest rate and the amount, e.g.
5000*10% gives answer of 50

Produce a number of prototypes to show the company (or sketch on paper if you don't get so far). Bear in mind it is a reduced version of the finished program, not the whole thing!

Note:

the interest rates can be 6, 7, 8, or 12 % If you use a combo box, you can put a series if items in its List property.
a 'print' facility is needed
can 'print' be performed at any time?
what initiates the calculation (a click, a key press...?)
what happens with negative or non-numeric input?

Here is how you can detect a non-numeric key entered in a text box:

   private sub txtBox_Keypress(KeyAscii as integer)
        if  ( KeyAscii < Asc("0")  or   (KeyAscii > Asc("9")  ) _
             and  KeyAscii <> Asc(".") then
           KeyAscii = 0
           Beep
        endif
    endsub

You can also set the 'enabled' property of a control to true or false.