This is cool. When it's in 'all operators' mode it's challenging!

One thing that I noticed is that there's a timer. Depending on your target audience, you might want to make that optional.

My son is 7 and finds the timer on Zetamac creates unnecessary stress. So I made something simpler for him that just shows a fixed number of problems (with no time limit), and uses a cookie to track the streak length: https://math.twilam.com/