# Quick question about semiconductor alloys and band gaps?

This seems like a straightforward thing but no textbook I've seen addresses it. Take the direct gap semiconductors GaAs (band gap 1.42 eV) and InAs (band gap 0.42 eV). If these two were alloyed together, would the band gap become some sort of average between the two values?

As an example, say we had Ga0.4In0.6As. First question, are the subscripts talking about relative mass densities or number densities of Ga and In in the alloy?

Secondly, do I get some sort of weighted average band gap for this alloy if I calculate

$$E_g = 0.4*1.42eV + 0.6*0.42eV = 0.82eV$$ ?
According to this wikipedia graph

http://en.wikipedia.org/wiki/File:InGaAs_Energy_band_composition.PNG

it appears to be about 0.7eV for a Ga fraction of 0.4. Am I calculating this the wrong way or what?