Rhodium-101 has a half life of 3.3 years. What percent of the sample will remain after 6.6 years?

25%

Explanation:

Half life means that 50% of the sample is gone at 3.3 years. This means that an additional 3.3 years (total 6.6 years) will reduce the sample a further 50% from the point at 3.3 years. In numbers, this means 50% of 50% (0.50*0.50), which is 25%.