How the heck do you figure this out!!?

Question

Radioactive decay of granite and other rocks in Earth’s interior provides sufficient energy to keep the interior molten, to heat lava, and to provide warmth to natural hot springs. This is due to the average release of about 0.03 J per kilogram each year. How many years are required for a chunk of thermally insulated granite to increase in temperature by 500°C? (Assume that the specific heat capacity of granite is 800 J/kg×C°.)

Answer 1

Let's use a kg - that'll make it easier.

So, a 500C increase requires 800J/degree x 500C or 400kJ

At 0.03J per year, this is 1.3x10^4 years.

That's 13,000 years