Calculation of submitted answers within the questionnaire

Hi there,
I have the following problem within my survey structure:
My survey participants have to type in a value in € (Text entry) - two questions later I want to give a further information, where I would like to automatically calculate the value, they have provided to me, multiplied by 0,8 (to get their inserted value minus 20% less).
I know that there is a code, to query an inserted value: ${q://QID17/ChoiceTextEntryValue} - but I do not know how to correctly adapt the code to multiply the TextEntryValue by 0,8.
First question: do I have to adapt the question type (I was using the Text entry option)?
Second: how exactly do I have to adapt the code to get the multiple (value * 0,8)?
I hope some of you can help! Thank you so much in advance!
Best, Carol