Just got this book yesterday in the mail: “Numerical brain teasers” from Pragmatic Programmers. Just in time for my multi-day stay at a hotel in Germany.
If you like fiddling with numbers and need some off-line time (or get bored quickly on the toilet), I can recommend this one. Enough challenges for weeks.
https://pragprog.com/titles/esbrain/numerical-brain-teasers/