Structures and $IUPAC$ names of some hydrocarbons are given below. Explain why the names given in the parentheses are incorrect.
$(a)$ $CH_3-CH(CH_3)-CH_2-CH_2-CH(CH_3)-CH(CH_3)-CH_2-CH_3$
Name: $2,5,6-$Trimethyloctane [and not $3,4,7-$Trimethyloctane]
$(b)$ $CH_3-CH_2-CH(CH_2CH_3)-CH_2-CH(CH_3)-CH_2-CH_3$
Name: $3-$Ethyl$-5-$methylheptane [and not $5-$Ethyl$-3-$methylheptane]

(A) According to the lowest locant rule,the set of locants $2,5,6$ is lower than $3,4,7$. Therefore,$2,5,6-$trimethyloctane is the correct name.
$(b)$ When substituents are at equivalent positions from either end of the chain,the lower number is assigned to the substituent that comes first in the alphabetical order. Since $E$ (Ethyl) comes before $M$ (Methyl),the name $3-$ethyl$-5-$methylheptane is correct.