I keep getting numbers mixed up?

i just need some general advice from anyone. i keep getting numbers mixed up at work and its really getting to me because i feel as though customers may think its on purpose but it honestly isn’t. for example, if the bill is £27 and they give me £40 in two 20 notes, i’ll end up giving them £3 instead of £13. its a genuine mistake and if they dont realise its wrong i wouldn’t know either. i have had some reasons to believe im dyslexic, and my brother has been diagnosed by it but i dont know if this is because of the dyslexia or because im just straight up stupid.

any advice on what i can do?
i might just write the change for the bill at the back before they come in just incase they pay cash. but sometimes it gets busy so i can't do that!!
I keep getting numbers mixed up?
Add Opinion