A sample of petrol is a mixture of 30% n-heptane and 70% iso-octane. The sample has octane number

The percentage, by volume, of 2,2,4-trimethylpentane(iso-octane) in that mixture is the octane number of the fuel.Petrol sample 30% n-heptane + 70% iso-octane since iso-octane is 70%. Hence, octane no. is 70.