chemistry

This is a lab question...I don't understand how to do it. A solution consisting of isobutyl bromide and isobutyl chloride is found to have a refractive index of 1.4194 at 20 degrees celsius. The refractive indices at 20 degrees celsius of isobutyl bromide and isobutyl chloride are 1.4368 and 1.3785, respectively. Determine the molar composition (in percent) of the mixture by assuming a linear relation between the refractive index and the molar composition of the mixture. From poking around the internet I got that I should be using this equation: (1.4194)(100) = 1.4368x + 1.3785y and x + y = 100 Obviously x + y = 100 is used because we are looking at % composition and it has to add up to 100. But I don't really understand the first equation.