Solve Vertical Subtraction: 91 Minus 90 Step-by-Step

To solve this subtraction problem, we will perform the operation vertically, step by step:

• Step 1: Write the two numbers in vertical alignment:
        91
  -   90
       ---
• Step 2: Start subtracting from the rightmost column (the units column):
• Subtract the units: $1 - 0 = 1$.
• Step 3: Move to the tens column:
• Subtract the tens: $9 - 9 = 0$.

So, after performing the subtraction, the result is $1$.

Therefore, the solution to the problem is $1$.