Which isomerism is exhibited by the functional group diethylamine?

Which of the following pairs represents metamers?

Compounds $P$ and $R$ in the following reaction are:

What type of isomerism is exhibited by neopentane $(C_5H_{12})$?

Compounds with the same molecular formula but differing in their structures are called structural isomers. What type of structural isomerism is shown by $CH_3-S-CH_2-CH_2-CH_3$ and $CH_3-S-CH(CH_3)_2$?

The compounds $(I)$ $HOOC-CH(OH)-CH(Br)-COOCH_3$ and $(II)$ $CH_3OOC-CH(Br)-CH(OH)-COOH$ are related as: