Solve these equations numerically - Stellar Abundance Ratios

[vmr101]

1. Question from a textbook.
I have written down the differential equations for part a) and shown part b),
but I am unsure of how to tackle part c).

2. This is the Question from the book
http://www.m-rossi.com/img/asp3012-1.png

Any advice would be grateful. Thank you

 

[mfb]

It is a numerical integration. Start with x=1, calculate the derivatives, estimate the numbers a small time step later, continue until the fraction of remaining helium is negligible.