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Adding decimal numbers involves a few simple steps. Let's use the example of -3.5 and 12.4 to understand this. First, align the numbers by their decimal points. This ensures that each digit is in the correct place value.
Ìý-3.5
  12.4
Next, focus on the part after the decimal point. If necessary, you can add zeroes to make the decimal places equal. Here, they both have one decimal place, so no adjustment is needed.
Once aligned, you add or subtract the digits column by column starting from the rightmost digit. Remember to carry over if a column adds up to more than 10. In this example, you're working with positive and negative numbers, making subtraction necessary.

When adding a negative number to a positive number, you are essentially subtracting the smaller absolute value from the larger absolute value.
For example, with -3.5 and 12.4:
The absolute value of -3.5 is 3.5.
The absolute value of 12.4 is 12.4.
Since 12.4 is larger, subtract 3.5 from 12.4:
12.4 - 3.5 = 8.9
The result takes the sign of the larger absolute value. In this case, it is positive because 12.4 is positive. Therefore, the sum of -3.5 and 12.4 is 8.9.
This process of using absolute values helps simplify the steps and keeps the focus clear.

Absolute value refers to the distance a number is from zero on the number line, regardless of direction. It is always positive or zero.
If you have the numbers -3.5 and 12.4, their absolute values are 3.5 and 12.4, respectively.
When dealing with addition of negative and positive numbers, understanding absolute value is crucial. It helps you determine which number has a larger magnitude.
In our example, when adding -3.5 to 12.4, instead of getting confused by their signs, you can compare their absolute values. Subtract the smaller from the larger to find 8.9, and then assign the sign of the larger number to the result.
Knowing how to work with absolute values makes operations with negative and positive numbers much simpler and more intuitive.