Skill: Find and correct a subtraction error

When you find a subtraction error, check each place value: ones, tens, and hundreds. Look for a place where regrouping was needed but was missed or done incorrectly.

Example:

$$\begin{array}{r} 542\\ -\ 268\\ \hline 324 \end{array}$$

The answer $324$ is incorrect. Let’s find the error.

1. Check the ones.
   We cannot subtract $8$ ones from $2$ ones, so regroup $1$ ten as $10$ ones:

   • $542$ becomes $5$ hundreds, $3$ tens, and $12$ ones.
   • $12-8=4$ ones.

2. Check the tens.
   Now we have $3$ tens, but we cannot subtract $6$ tens from $3$ tens. Regroup $1$ hundred as $10$ tens:

   • $5$ hundreds becomes $4$ hundreds.
   • $3$ tens becomes $13$ tens.
   • $13-6=7$ tens.

3. Check the hundreds.

   • $4-2=2$ hundreds.

So the correct difference is

$$\begin{array}{r} 542\\ -\ 268\\ \hline 274 \end{array}$$

The error was in the tens place: the $3$ tens needed to be regrouped with $1$ hundred before subtracting $6$ tens.

Check the answer with addition:

$$274+268=542$$

Because the sum matches the starting number, $274$ is the correct difference.