actually we start numbering by minimising the number of highest order addition, which is the isobutyl, if it gets same number regardless, then we try to minimise the sum of numbers, so i think it should be called
5-(isobutyl)-5,6,6,7,8-penta-ethyl-8-methyl-decane (I am assuming hydrogen’s are present, just not represented, because that usually is the case)
I may alo be wrong here, it has been 4 years since I have been required to do nomenclature myself
Now i remember we also had to specify what isobutyl (technically just 1 isobutyl exists (not counting stereoisomers), and other form would be tertbutyl). But giving highest priority and minimising sum were definitely something we were taught.
I was free enough to look it up this time - IUPAC guidelines for organic chem - https://iupac.org/wp-content/uploads/2021/06/Organic-Brief-Guide-brochure_v1.1_June2021.pdf (or more generally https://iupac.org/what-we-do/nomenclature/brief-guides/)
Section 7 ( c ) Lowest locant(s) for principal characteristic group(s)
Although I also remember just as we completed our unit on nomenclature, all we got was “common names”, now i was supposed to know of the top of my head what a cumene is (which I think is isopropyl benzene (not going to check this one)). Same thing happened with polymers, we were taught IUPAC, and then again, “industrial names”