Why is the melting point of #"isobutane"# LESS than that of #"n-butane"#?
Because the degree of intermolecular force is LESS in isobutane than in butane........
And the degree of intermolecular force is also illustrated by the respective boiling points (which are?). Long chain alkanes can interact chain to chain by (fairly weak) intermolecular dispersion forces. The longer the chain, the greater the intermolecular force, and the more elevated the melting point, and the boiling point.
And what are the normal boiling points, and melting points of butane, and isobutane? I don't know, but you need to know.