2-Methylbutane

Take a look at the direction of numbering

2-Methylbutane

Numbering can also be done like this

2-Methylbutane

4-Ethylheptane

4,5-Dimethyloctane

4-Methyl-5-ethyloctane (wrong)

Why?

4-Ethyl-5-methyloctane (correct)

Alphabetical Order : From either side, we get first alkyl group at 4^th carbon. Since E comes before M, preference is given to the ethyl group.

4-(1-Methylethyl)heptane