1. The problem is to calculate $10 - 20$. 2. The subtraction formula is $a - b$, where $a$ and $b$ are numbers. 3. Here, $a = 10$ and $b = 20$. 4. Subtracting $20$ from $10$ means moving $20$ units to the left on the number line starting from $10$. 5. Calculate $10 - 20 = -10$. 6. The result is $-10$, which is a negative number because $20$ is larger than $10$.