I pick two whole numbers x and y between 1 and 10 inclusive (not necessarily distinct). My friend picks two numbers x - 4 and 2y + 1. If the product of my friend's numbers is one greater than the product of my numbers, then what is the product of my numbers?
We have: (x-4)(2y+1)-1 = xy
So xy + x - 8y - 5 = 0
The numbers are 2 and 7 so 14.
See the detailed answer here: https://web2.0calc.com/questions/i-pick-two-whole-numbers