Puzzle Box, Volume 1 (Dover Brain Games & Puzzles)

Multiplying my age by 6 then subtracting 6 produces the same result as subtracting 7 from my age then multiplying by 7. How old am I?
On my broken calculator with keys + - ÷ x =, the only functional number is 7. How can I get 34 to appear in the readout?
A country mints four denominations of coins, in whole numbers of cents. It takes four of these coins to make 21¢, or 24¢, or 25¢, or 26¢. What are the denominations of the coins?
These and almost 300 other mathematical puzzles appear in this original collection, devised by world-renowned mathematicians, puzzle creators, and devoted puzzle lovers. A unique puzzle project, it unites the efforts of a dozen authors, including software engineer Andrea Gilbert and Bram Cohen, author of the P2P BitTorrent protocol.
Seventeen different types of challenges include 3-D puzzles, chess puzzles, connections, dissections, foldings, geometrical puzzles, logic problems, matchstick puzzles, mazes, moving pieces, number puzzles, put-togethers, strimko, sudoku, visual puzzles, weightings, and word puzzles. The difficulty level of each puzzle is marked by stars, ranging from 2 to 5. Average difficulty level is about 3 stars, promising puzzle enthusiasts many entrancing hours of solving and enjoyment.

Peter and Serhiy Grabarchuk are part of the Grabarchuk Puzzle Family. Over a period of decades they have produced puzzle books, puzzle apps, interactive puzzles, and more.