CGPET 2015 Solved Paper

Gasoline mainly has compounds from C5−C10 range. Gasoline is used in spark-ignition (SI) engines. Octane number is a measure of the anti-knocking capacity of the fuel. 2, 2, 4 trimethylpentane (Iso-octane) has been designated an octane number of 100 and n-heptane has been assigned an octane number of zero.
This sample of gasoline contain 81% octane. Hence, it's octane number will be 81.