If familiarity with foreign languages marks one out for suspicion (it does) then familiarity with numbers is equivalent to witchcraft.
A few years back, when Sainsburys were running their "round up your bill for charity" initiative, I found myself with a bill of £x.67. Feeling unusually charitable, I said yes to rounding up, and when the checkout operator asked "How much ?" I said "33 pence, please".
"Ooh" they said. "That's lucky it's exactly £20.00 (or whatever)".
"No luck", I said. "I worked it out".
"No" they said "no one can do that."
I was just about to say something more when an older gentleman behind me in the queue gave me a stern rebuke for lying to the young person and it wasn't a nice thing to do.
Discretion being the better part of valour, I apologised.