Oh, math is everywhere and informs, either directly or indirectly, every part of our life!.
Think of letters as substitutions. Do you play any sports? It is common for one player to 'sub in' for another. Sometimes it is to give a player a break (from running up and down the field/court constantly) sometimes it is because they need someone with a particular skill at the moment. Letters in math allow you to work with out having a set value - it is how we use the math to figure out what the number SHOULD be given the situation. So, a letter is like stating a position, without stating the player's name (like center for Basketball, or Goalie for hockey or soccer)
Think about this - you are shopping and want to know what your bill will be - you don't know the answer until AFTER you calculate it. With Algebra, you can input a bunch of data and find the best answer (think of a business - input the hourly rate of an employee, the hourly cost to run equipment, and the hourly cost of materials...and the hourly output of equipment and personnel. That helps them determine the selling price of the item they are producing.
Numbers tax:
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My molting Roo - counting his ladies to make sure that they are all there!