Final answer:
The number used to indicate the double bond position for CH3-CH2-CH=CH-CH3 in IUPAC nomenclature is '2-', resulting in the name 2-pentene.
Step-by-step explanation:
The number that would be used to indicate the double bond position in the IUPAC name for CH3-CH2-CH=CH-CH3 is determined by numbering the longest carbon chain containing the double bond so that the double bond receives the lowest possible number. According to IUPAC rules, the carbons are numbered starting from the end of the chain nearest to the double bond. In this case, the double bond is between the second and third carbon atoms.
Therefore, the IUPAC name for this compound is 2-pentene, where 'pent' indicates the five carbon atoms in the longest chain and 'ene' indicates the presence of a double bond. The number '2-' before 'pentene' shows that the double bond starts at the second carbon atom. Remember to provide the lowest possible numbering for the location of the double bond according to IUPAC nomenclature.