PGM1 mode. The /FOR button is the multiplication button next to the decimal point. If you have the european version, it is often labelled 'X' (sometimes with a circle around it).
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If anyone needs a Sharp ER-3110 manual, one of the users has listed it here:
http://the-checkout-tech.com/manuals/cashregisters/Sharp4.php
Registerman, March 2014