Say you have a bag with either a white counter or a black counter in it, but you don't know which. Then you drop a white counter into the bag. Shake it up and remove one counter. It's white. What are the odds that the remaining counter is also white? If you enjoy puzzles like this, then Ball's collection of number games and logical conundrums is for you.
The second half of the book contains not just answers but full, easily comprehensible explanations: a fun way to smuggle some mathematical principles to children and indeed adults. And it's ideal for passing the time on long family car journeys. The jokes are pretty bad, though ("Do house hunters use a very large net?"). By the way, the odds of the second counter being white are 2 in 3. For an explanation, you'll have to see the book.