How can I explain the formula and name of condensed formula #(CH_3)_2C(C_2H_5)_2# according to IUPAC?
First, you recognize that one of the CH₃ groups and one of the C₂H₅ groups is part of the main chain.
Then you can re-write the condensed formula as CH₃-C(CH₃)(C₂H₅)-C₂H₅.
You can expand the right hand C₂H₅ group to get CH₃-C(CH₃)(C₂H₅)-CH₂CH₃.
The structural formula is then
When you number the main chain from the left hand end, the IUPAC name becomes 2-ethyl-2-methylbutane.