Drop down menus to apply different Formulas

[bollo]

Hi

Hope someone can assist. I have made an mortgage calculator for monthly repayments , what I’d like to include is a drop down list that you can also choose fortnightly and weekly options and by doing so it will adjust the formula and recalculate.

I'm a bit of a beginner when it comes to excel and I can't figure out how to go about this without making 3 separate calculators ..any help much appreciated
</SPAN>
Paul

 

[bollo]

Sorry maybe i wasn't clear enough in first post. How can i switch between different formaula with a dropdown here is an example the below would be used for monthly

=IF(G24>0,(NPER(G15/12,G26,K22))/12,(NPER(G15/12,G26,K22))/12)

and then this one for fortnighly

=IF(G24>0,(NPER(G15/26,G26,K22))/26,(NPER(G15/26,G26,K22))/26)

I have already made 3 calculators monthly forthinghtly and weekly but i want to combine them into one
calculator

Is it somthing like conditional formating i'm looking for?