Octane number is defined as the percentage by volume of iso-octane present in the mixture of $\mathrm{i}\mathrm{s}\mathrm{o}-\mathrm{o}\mathrm{c}\tan \mathrm{e}$ and $\mathrm{n}-\mathrm{h}\mathrm{e}\mathrm{p}\tan \mathrm{e}$ which has the same antiknocking properties as the fuel under examination.

Given fuel (25% n-heptane +75% iso-octane) Hence, octane number = 75 (because iso octane is 75%)