CO2 and Coal

Assume that the total coal proved reserves of the world are eventually burned and that 50% of the resulting CO2 is added to the atmosphere. Estimate the resulting increase in concentration of CO2 (in ppm by volume or, equivalently, by number of molecules) in the atmosphere of the world. Assume that the coal is 100% carbon. Given: 948*10^9 tonnes of coal 5.7 * 10^15 mass of atmosphere. I know the answer is 230ppm, but need to know HOW to solve it.