How come Mr Bent came to the bank aged 13, stayed for thirty nine years, and yet is specifically said to be in his mid to late forties by characters who should know? Writers Cannot Do Math?
When is that stated? Moist very clearly states once that he's 52.
I can't remember for certain, but I'm pretty sure that it's just mentioned that he appears to be mid to late forties, which by no means is a certain statement of his real age.
Corgi Books 2008, paperback with a 25 years of Discworld sticker , Moist says "he's forty seven" some pages after the mention of thirty-nine plus thirteen. Probably an error that got past editing.
Or Moist isn't able to do math as quickly as Bent, certainly not when his mind is otherwise occupied.
If there are Ankh-Morpork coins worth a fraction of a cent, doesn't that spoil Arnold Sideways' favorite begging technique of asking people "Do you have change for a penny?"
Not necessarily- there are several countries where the penny got phased out of use simply because people didn't use them as much/it was inconvenient. It may be a similar thing here- most people don't use fractions of pennies. Alternately, Arnold's usual targets are the ones who spend in denominations greater than pennies.