Answer:
The mistake is that the denominators are not the same in these fractions so you cannot add them yet. So, you would have to change the denominators to be the same before adding. The denominators I chose is 15, 2/5 times 3 which is equal to 6/15 and 1/3 times 5 which is equal to 5/15. Now, we can add 6/15 and 5/15 which is equal to 11/15!
Hope this Helps! :)
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-SGO