Translated (by me) from a Swedish science magazine I bought when I flew back to the UK a couple of days ago:
"How old is your dad?" Karin asked, "he looks much older than my dad". "Ridiculous!", Betty replied somewhat angry over her friends insensitivity, "but now you'll see if you can work it out. His age is three times the first digit plus five times the last." How old is he?
If you can work it out. Don't just tell me an answer you've come up with by testing several numbers. Show me some proof!