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What is the name of this $\ce{C13H28}$ isomer:

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I think it must be "4-ethyl-6,7-methylnonane," but I saw somewhere that its name is "6-ethyl-3,4-methylnonane". Why? The ethyl number must be smaller, isn't it? (because E is before M). Am I wrong?

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Alphabetic order dictates which order the substituents are written in, but not the numbering of their positions. Numbering is determined by what minimizes the numbers. E.g. 3-ethyl-2-methylhexane is correct, but 4-ethyl-5-methylhexane is not.

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