Work Step by Step
First, you have to calculate how many half-lives are required for it to reach 17 counts per minute. Simply divide 544 counts in half again and again until you reach 17. Therefore, 5 half-lives is required. 30 years times 5 half lives = 150 years + 1985 = 2135. The half-live is given by 30 years. Multiply that by 5 half-lives to get 150 years and add that on to the given date of 1985.