Dylan's (absurdly) poor maths 'skills'.

Has anyone ever encountered someone with such weird maths 'skills' as Dylan?

Even people who are really bad at maths don't usually come up with the sort of answers he did because we have an innate ability to estimate which tells us that wildly inaccurate calculations are wrong. Even if for some odd reason Dylan lacked this ability he would surely have learned during childhood that the type of answers he came up with would make him look like a cretin and would either always use a calculator or leave such calculations to someone else.

People who hold down the type of jobs he did are very adept at hiding an inadequacies and knowledge gaps and it seems incredibly unlikely that he's want people knowing he was not only useless at maths but didn't even have the intelligence to know that his answers were beyond stupid.