I don't have autism myself, but I'm terrible with numbers, except few main phone numbers. I forget them as I literally hear them. I tried improving this by playing little Ds games that make you memorize numbers lol. Those type of games were fun and sometimes helped me a bit with practice, but I still have the issue. I also mix up my numbers. I also have concentration issues.
My bf with "Autism" traits, is amazing with numbers, he reads a license plate and is like bam in his head. My boyfriend gets distracted by things, and it really irritates him as he loses his place, especially when coding/doing computer stuff/compiling So he uses music. It's funny, teachers would always yell , music distracts you blah. Actually for my boyfriend it allows him to concentrate. Instead of getting distracted by 100 things in the room, he will get distracted to the music, when his mind wanders, then he goes back, that way it's loss of concentration between two things, instead of 100s.
But like people are saying, some people are actually autistic with amazing creative skills, and are artists, but arent really great with math. Everyone has their strong points and weak points in life, I'm sure you have your own set. But you can probably improve these things a tiny bit.