Represent your numbers as follows.
X ⇒ first number
X + 1 ⇒ second number
Since their sum is 91, we have x + (x + 1) = 91.
Solving from here, simplify on the left side to get 2x + 1 = 91.
Subtract 1 from both sides to get 2x = 90.
Now divide both sides by 2 and x = 45.
So if x = 45, then x + 1 must be 46.
So our numbers are 45 and 46.
