What is the difference between isopentyl and sec-pentyl?

Please explain how to solve (e)

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Draw the structures of the following groups, and give their more common names. (a) the (1-methylethyl) group (b) the (2-methylpropyl) group (c) the (1-methylpropyl) group (d) the (1,1-dimethylethyl) group (e) the (3-methylbutyl) group, sometimes called the "isoamyl" group 37) PROBLEM 3-7 Draw the structures of the following groups, and give their more common names. (a) the (1-methylethyl) group (b) the (2-methylpropyl) group (c) the (1-methylpropyl) group (d) the (1,1-dimethylethyl) group (e) the (3-methylbutyl) group, sometimes called the "isoamyl" group A) C) YR2=IS0 pwpyl R=propyl E)