A cyclopentane ring carries a methyl group and a 2-methylpropyl group on carbons that are in a 1,3 relationship. What is the IUPAC name of the compound?

Both ways of numbering give the locant set {1,3}. The tie is broken by giving locant 1 to the substituent cited first in alphabetical order. 'Methyl' precedes 'methylpropyl' (the shorter name comes first when the letters are otherwise identical), so methyl gets C1. The prefixes are written alphabetically: 1-methyl-3-(2-methylpropyl)cyclopentane.