If a and b must be positive integers, what is the largest integer n such that 13a+13b=n has no solutions?

13a+13b=n

13(a+b)=n

Smallest Possible Positive Integer = 1

The question does not mention that a and b should be the same so I will set a and b both 1

13(1+1)=26

(The Greatest Integer Smaller Than 26) = 25

The largest integer n such that 13a+13b=n has no solutions is 25.