I think the digits should be arranged 0 to 9, not 1 to 9 followed by 0. Because 0 is less than 1.
Also, when the correct answer is revealed, it would be great to somehow highlight the cats that are actually described by the formula. Yes, this is hard for 0.
Also, the animations feel too fast to me. I find they make the whole thing stressful, somehow.
ALSO, when a wrong answer is given, the question disappears! That's terrible UX.
EDIT: I should add that I think the idea is great! But some details of the implementation could be improved.
Also, when the correct answer is revealed, it would be great to somehow highlight the cats that are actually described by the formula. Yes, this is hard for 0.
Also, the animations feel too fast to me. I find they make the whole thing stressful, somehow.
ALSO, when a wrong answer is given, the question disappears! That's terrible UX.
EDIT: I should add that I think the idea is great! But some details of the implementation could be improved.