Hello Markus,

When there is a data written without explanation like this, you can consider it as a data that you can ask your interviewer.

Anyway I agree with you: it is a complex and crucial data and it's weird to directly ask your interviewer that value. In order to make a quick estimation of that number I would divide the population in segments by age, estimating the percentage paid by credit card per each group (I expect that 20-40 years old use credit card more often than people over 40).

Then you can also add some complexity and state some further aspects to be considered, such as the average income of each segment. In fact 40-60 y/o people have an higher purchasing power than younger people, so that segment should weigh more in the total average.

Does it make sense?

Hope it helps,
Luca