Today's terrestrial background radiation is about 50 mrem/year, mainly due to the uranium and thorium decay chains, and potassium. The earliest evidence of life on earth was about 3.5 billion years ago. If the approximate current contributions for each decay chain is Th-232: 11 mrem/year, U-238: 26 mrem/year and K-40: 11 mrem/year, what was the average dose rate 3.5 billion years ago? State the source of all values used in your calculation. Please show all work.