i have two values toleranz.values = 0.125 pruefmittel_wert.values = 0.3

In the office.js api i get a string as var xmp01. How can i change the formula to get a value for xmp01T. I made it with += but did not get a result

var xmp01T = 0.1 * toleranz.values + pruefmittel_wert.values;