The same way you add any other number that has a unit attached.
Example
10% + 3.6% = 13.6%
_____
You need to be a bit careful with this. You asked about adding percentages, and you got the correct answer for adding percentages.
Often, a situation arises where it looks like you might want to add percentages, but that would be an error. Consider a discount on a discount, for example. If you take 50% off the price, and then another 50% off that, you end with 75% off, not 100% off.
Example
10% + 3.6% = 13.6%
_____
You need to be a bit careful with this. You asked about adding percentages, and you got the correct answer for adding percentages.
Often, a situation arises where it looks like you might want to add percentages, but that would be an error. Consider a discount on a discount, for example. If you take 50% off the price, and then another 50% off that, you end with 75% off, not 100% off.
