I have a question in epidemiology and I am not sure which category to post it in.

The death rate per 100,000 for lung cancer is 7 among non-smokers and 71 among smokers. The death rate per 100,000 for coronary thrombosis is 422 among non-smokers and 599 among smokers. The prevalence of smoking in the population is 55%, so what is the etiologic fraction of disease due to smoking?

I missed this on the homework and I need to know how to get the correct answer before finals. I just know a similar question will be one the exam since most of us missed this one. Please show me in detail how you get the answer so that I can apply it to other similar problems.