At 500C, the temperature has dropped 100 degrees from 600 C. That is 10*10 so the time, starting at 1 second, is doubled 10 times. "Doubling 10 times" is .

At 650C, the temperature has increased by 50 degrees from 600 C. That is 10*5 so the time, starting at 1 second, has halved 5 times. "Halving 5 times" is .