How do I get an IF function to return a percentage when applicable?

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Here is the problem:

I am using a Concatenated Lookup function in conjunction with a drop down
box. There are three options in the drop down - two are numbers, one is a
percentage.

I need to get the lookup funtion combined with an IF function to say when I
choose one of the two number options return a number, when I choose the
percentage option, return a percentage.

Any ideas? I looked for a PERCENT funtion but there isn't one!

This one has me stumped.

Thanks

Simon