There was a programme where primary-school kids were taught Esperanto as their first foreign language for a year, before switching to French for N years. Not only did those in the programme speak better French at the end than those who had learnt French for the whole N+1 years (which was the hypothesis being tested) but it was also found that many of the kids translated arithmetic problems into Esperanto before solving them and translating the answers back into English.
5 is kvin and 10 is dek so 50 is kvindek. If you know how to count to 10, you know how to count to 99.
100 is cent, so 500 is kvincent. 505 is kvincent kvin and 555 is kvincent kvindek kvin. Alright, now you know how to count to 999.
A "number to Esperanto" is a great exercise to code and Esperanto is a lovely language to learn. Try it!