A radioactive substance has an initial mass of 500 grams and a half-life of 10 days. Which equation can be used to determine the number of days x required for the substance to decay to 321 grams? a)500 = 321 (1/2)^x/10 b)321 = 500 (1/2)^x c)321 = 500 (1/2)^x/10 d)500=321 (1/2)^x