So fast, can't think

I wondered what kind of math question you'd show and how exactly the difficulty would increase. Unfortunately, it's all just additions and subtractions with a slightly larger numerical range over time, which isn't really that impressive.
I'd introduce new operations – let's say, every fifteen questions: multiplication, division, fractions, or go all out at later stages with stuff like "sin 90° = 1", "log2 (8) = 3", "derivative 2x = 2" etc. It would make the game more interesting because of the bigger variety and by making people wonder what comes next, thus keeping them engaged for longer.

It also needs some balancing. I got to question #25 until I got bored and started clicking randomly without even looking at the question, which got me safely to question #63 when I ran out of time (and could probably go further if I clicked faster).
Consider these options:
1) Increase time deducted for a wrong guess. While it's very good that you don't lose after the first wrong guess, it shouldn't be possible to try all four options before your time limit runs out.
2) Use a "total time" bar shared by all questions. A successfully answered question on the first try increases the timer, a wrong answer does nothing – thus if you make a mistake, you have to make up for it with a chain of correct answers.

But mainly, endless games need to introduce new elements to still simulate the sense of "progress". This doesn't have it (yet).

Simple background design, alright music choice and sound effects, you might want to have the game start off with music and sound though as I had to toggle them on. Really cool actually, my high score was 78

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This is totally not my taste. But that's ok. Not really a game IMO
If you want to make a game out of simple math problems, then make it fun.
Also if you decide to put a very loud & invasive music track into your game, please provide an option to mute it (during gameplay)

What is your goal with this game?