(1-Methylcyclobutyl)methyl bromide is solvolysed in aqueous ethanol. Taking the carbocation pathway that ends at the tertiary cation, the principal alcohol formed is

Ionisation (or concerted migration) at the primary carbon is accompanied by migration of a ring C-C bond to the $\mathrm{CH}{}_2$ carbon, expanding the four-membered ring (relief of ring strain) to a five-membered ring. The positive charge ends on the carbon that carries the methyl group, giving the tertiary 1-methylcyclopentyl cation, which is trapped by water to 1-methylcyclopentan-1-ol.