Submit a variable as anwser to a fill in the blank

We would like to create an interaction where users can enter a correct answer using number buttons on screen, just like a calculator. So I have a variable 'currentNum' which has the value of the number that a users types using those buttons. It looks like this:

 So when a user clicks the buttons 2 and 3 in that order the value of %currentNum% becomes 23. In a normal text entry box this is displayed correctly. However, when I enter the same reference into the 'fill in the blank' text entry field so that the interaction would recognize '23' as my answers, it doesn't seem to work.

How can I get the fill in the blank interaction to recorgnize the value of my variable as an answer and grade it against the accepted answers?

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