how to do 15 lesson

[AliuxALiux]

sorry, not issue: i am not good know english language, but i didn't know how to do lesson 15 (the remainder of the division). Russian version of Learn GDScript From Zero. 15 lesson - the practice is to do dice roller, but variable "sides" not changeable, is need it is necessary that the range of numbers from 1 to 20 remain for the dice, this should be remainder