How to buy tickets for U-bahn

Buying tickets for U-bahn, Frankfurt's metro, is horribly tricky for first-timers. Here's what I learned from my friends living there.

For a single ticket within the city center, enter 97 by using number keys. Then press the green button with letters "97 Kurzstrechke". Then pay 1.50 euro.

For a one-day travel card, enter 5000 by using number keys. Then press the yellow, second-to-top button with letters "Tageskarte". Then pay 5.80 euro.

Here's a booklet in English, which says you don't need to enter 97 for a single ticket within the city. I don't know which is correct.

If this is too complicated, forget buying a ticket and you'll run a (rather small) risk of paying the penalty 40 euro if you encounter a ticket inspector. (There's no ticket barrier.)