Increase the number of random items selected from a list in excel

I want to build a 6 level task, at each level a random letter of the alphabet is selected and then the subject is asked to solve a sum. For each level the number of letters that are selected increases by 1 (Level 1 - 1 letter, Level 6 - 6 letters). I guess I have to make a loop for each level, but how do I make it so that instead of selecting one letter I can now select two, three, …, six different letters randomly from my list for each trial of each level.

Try something like this (I’m not sure how you intend to use the letters.

Begin Experiment

letters = ['A','B','C','D','E','F','G'...]

Begin Routine

theseLetters = letters[:trials.thisN+1]