It is in alphabetical order - before propyl. The butyl and propyl substituents are equidistant from the dimethyl position. The first thing to consider is proximity of substituents - there are 2 methyl groups on the same carbon.
What confuses me is that each substiuent is as far away from the next one, as all the others - there is a difference of two jumps between one substituent and its neighbour.
What I still dont get is why butyl is 3, when it should be 1 since it is alphabetically first]
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Has it got anything to do with the total number in the name
Currently: 1+1+3+5
Otherwise: 1+3+3+5
So currently the total of all the numbers in the name is the smallest possible.
Then, since butyl comes alphabetically next it is 3