What are some algebra riddles?

Algebra riddles can be either word problems that need to be translated into a set of algebraic equations or arithmetic problems with letters where each letter represents a digit, and the objective is to replace each letter with an appropriate digit.

Consider a few algebra riddles that are arithmetic problems with letters. Solve for each letter, where a five-digit number multiplied by 3 is a number with its digits in reverse order (ABCDEF x 3 = BCDEFA). Solve for each letter where the product of two three-digit numbers is 123456 (ABC x DEF = 123456) knowing that A = 1. Solve for each letter where the sum of two four-digit numbers can be represented by CHOO + CHOO = TRAIN.