When students learn how to add and subtract decimals, they often make some common mistakes. These mistakes can make it hard for them to understand the topic and get the right answers.

Aligning Decimal Points
One big mistake is not lining up the decimal points. It's really important to make sure the decimal points are in a straight column. If they're not, students can get the wrong answers. For example, when adding $2.5$ and $1.75$, it should look like this:

  2.50
+ 1.75
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If students just line up the numbers without paying attention to the decimal points, they might mix them up and get the wrong total.

Inconsistent Decimal Places
Another common error is not using the same number of decimal places. Sometimes, students forget to add zeroes when the numbers have different lengths. For instance, instead of $3.2$, it should be written as $3.20$ to keep everything lined up. This makes adding or subtracting much easier.

Carrying and Borrowing
Students might have trouble with carrying over or borrowing when it’s needed. If they don't understand this well, they can make mistakes with their math. For example, when subtracting $5.75$ from $6.2$, they should first change $6.2$ to $6.20$ before doing the subtraction:

  6.20
- 5.75
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Rounding Errors
Finally, students often round decimals the wrong way, especially in problems with many steps. This can cause even more mistakes later on. Teaching students to focus on being accurate instead of just guessing can help them get better at working with decimals.

By fixing these common mistakes, students can gain a strong understanding of adding and subtracting decimals. This, in turn, will help them do better in math overall.