Nucleocosmochronology: hydrogen/helium ratio and its change

[Logarythmic]

Homework Statement

Assume that the mass-to-light ratio, M/L, for the galaxy is, and has always been, 10 in solar units. What is the maximum fraction of the total mass that could have been burnt into helium from hydrogen over $10^{10}$ years? (The mass deficit for the reaction $4H \rightarrow ^4He$ is 0.7%)


2. The attempt at a solution
"10 in solar units" should mean

$$\frac{M}{L} = 10 \frac{M_{\odot}}{L_{\odot}}$$

but then what? This is a basic nuclear physics problem, isn't it?

 

[Logarythmic]

Can I use

$$\frac{N}{N_0} = e^{-\lambda t_s}$$

and put $t_s = 10^{10}$? If so I need to know the "half-life" for the proton-proton reaction. But there is something with the mass deficit aswell... As you can see, I'm not too good at nuclear physics.