Dimethylpentane:

The prefixes di-, tri-, tetra- etc. are employed for similar substituents, but the order within which they are written is still relies upon the alphabetical order of the substituents themselves (that is ignore the di-, tri-, tetra-, et cetera). For instance, the structure in below figure is termed as 5-ethyl-2,2-dimethyldecane and not 2,2-dimethyl-5-ethyldecane.

Figure: 2, 2-Dimethylpentane.

Diagram:  5-Ethyl-2, 2-dimethyldecane