chemistry question!

2-Methylbutane is reacted with chlorine by radical substitution.
What is the number of structural isomers with the molecular formula C5H11Cl that could be
formed?
answers 4 but I cant think how? thanks

If you draw the structure of 2-methylbutane out, you should find it is CH3CH2CH(CH3)2.

Because there is only a single chlorine added by substitution, there are 4 possible places the chlorine could substitute a hydrogen on to form distinct (i.e different) isomers.