${\mathrm{C}}_5{\mathrm{H}}_{10}$ does not show geometrical isomerism. Reaction of ${\mathrm{C}}_5{\mathrm{H}}_{10}$ with NBS in presence of sunlight gives two monosubstituted products. One of the products show geometrical isomerism. If the free radical intermediate undergoes rearrangement, then a tertiary and two secondary (optical isomers) monobromo products are formed.

If x = Number of alpha hydrogen atoms present in ${\mathrm{C}}_5{\mathrm{H}}_{10}$
y = Number of enantiomers formed in the reaction, then the value of (x-y) is