The subscript 3 outside of the parentheses means to multiply everything in the parentheses by 3, so it becomes a matter of addition:
3*(CH3) * (1 carbon atom)/(CH3) = 3 carbon atoms in (CH3)3
CH has one carbon atom and one hydrogen atom, so:
1 carbon atom in CH
Add both of them together, and you get 4 carbon atoms in isobutane