Some problem collections by Arnold:

* Problems for Children from 5 to 15: http://jnsilva.ludicum.org/HMR13_14/Arnold_en.pdf

* A mathematical trivium pt. 1: http://www.physics.montana.edu/avorontsov/teaching/problemof...

* A mathematical trivium pt. 2: https://www.latroika.com/mathoman/exos/arnold-exos-math-engl...

Thank you, Is there an answer key to those?

Not that I know of, but many of the problems have been discussed online, e.g, calculating the average of the 100th power of sin in under 5 minutes [1], which according to Arnold, if you cannot solve, you don't understand mathematics.

[1]: https://math.stackexchange.com/questions/24533/find-the-aver...