No control over the speed.

I dont know how it works on a regular computer. I have an iPad and the found the function symbols in the circles difficult to distinguish from each other at high speed. This caused the times to be slow, or they felt that way. One suggestion would be to put the function symbols in one box, thats been divided into four buttons, so you could see all of them at once. Youd have to put all symbols and numbers in some extra bold font. Ideally it would be better if you could see both the numbers and the answer box at the same time.

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