If a and b must be nonnegative integers, what is the largest integer n such that 13a + 18b = n has no solutions?
The answer is 25. See https://web2.0calc.com/questions/if-a-and-b-must-be-positive-integers-what-is-the-largest-integer-n-such-that-13a-13b-n-has-no-solutions
