Math questions published in a book are copyrighted as any other published text. Presumably, purely numerical questions are too.
Slight modifications to copyrighted works is usually considered derivative work, and if done without the author's permission is considered infringement.
My question is whether slighly modified math questions are considered derivative as well, or would it be original/transformed work, considering that merely changing the numbers in a math question would usually create a new problem to be solved. And TBH is probably how many, if not most, math questions for pedagogical purposes are generated.
A tap on the top of a 20L tank fills it at the rate of 4 liters per hour, and another at the bottom empties it at 2 liters per hour. If both taps are open simultaneously, how long will it take to fill up an empty tank?
A faucet on the top of a 40L tanker fills it at the rate of 6 liters per hour, and another at the bottom empties it at 1 liters per hour. If both faucets are open simultaneously, how long will it take to fill up an empty tanker?