Naming this oxo-carboxylic acid

On a textbook question, the answer key revealed this structure to be 3-methyl-2-oxopentanoic acid:


[![enter image description here](https://i.stack.imgur.com/5e2f8.png)](https://i.stack.imgur.com/5e2f8.png)


I found this strange, as the carbon from the left (#3) is methyl, but the oxo prefix is on the right for the $\ce{C=O}$. Why is this?




Starting from the group with the highest priority, the carboxylic acid group, we label the longest carbon chain:


[![enter image description here](https://i.stack.imgur.com/qTYAT.png)](https://i.stack.imgur.com/qTYAT.png)


This molecule might otherwise be named a ketone, but since the carboxylic acid group has a higher priority, it is treated as another substituent (oxo), located at the 2 position. The methyl group is attached to the 3 position, hence the name 3-methyl-2-oxopentanoic acid.


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