Curtis measured the temperature of water in a flask in Science class. The temperature of the water was 35°C. He carefully heated the flask so that the temperature of the water increased about 2°C every 3 minutes. Approximately how much had the temperature of the water increased after 20 minutes?

The water temperature increased by about 2° every 3 minutes, or of a degree every minute. Multiplying the increase in degrees per minute by the total number of minutes yields
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Since the problem asks for the increase in temperature and not the total temperature that results after the increases, 13 is the closest to our answer.