You should use Expression Manager. Have a look at the manual pages about it.
What you would do is create a equation type question and then multiply the value of the users response and save it in the equation question (which you can hide).
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Provide your LS version and where it is installed (own server, uni/employer, SaaS hosting, etc.).
Always provide a LSS file (not LSQ or LSG).
Note: I answer at this forum in my spare time, I'm not a LimeSurvey GmbH employee.
Not sure what you are trying to do. The number given by the respondent doesn't hurt you. So I would just leave it and ignore it during analysis. However, if you really must change it as well, you might be able to write the newly calculated value back into the question where the respondent gave the answer (probably possible via EM, you'll need to try it yourself, maybe something like questioncode==value). But I personally wouldn't recomend it, because if something goes wrong with the expression at one point during the survey, you will not be able to recalculate the value from the originally given value anymore. So I personally would keep them both.
Help us to help you!
Provide your LS version and where it is installed (own server, uni/employer, SaaS hosting, etc.).
Always provide a LSS file (not LSQ or LSG).
Note: I answer at this forum in my spare time, I'm not a LimeSurvey GmbH employee.