Structures and IUPAC names of some hydrocarbons are given below. Explain why the names given in the parentheses are incorrect.
(a) `CH_(3)-underset(CH_(3))underset("| ")(CH)-CH_(2)-CH_(2)-underset(CH_(3))underset("| ")(CH)-underset(CH_(3))underset("| ")(CH)-CH_(2)-CH_(3)`
2,5,6- Trimethyloctane
[and not 3,4,6-Trimethyloctane]
(b) `CH_(3)-CH_(2)-underset(CH_(2)CH_(3))underset("| ")(CH-CH_(2))-underset(CH_(3))underset("| ")(CH)-CH_(2)-CH_(3)`
3-Ethyl-5-methylheptane
[and not 5-Ethyl-3-methylheptane]

Explanation

(a) Lowest locant numbe 2,5,6 is lower than 3,5,7 (b). Substituent are in equivalent position, lower number is given to the one that comes first in the name according to alphabetical order.