What percentage of strontium-90 remains in a sample after three half-lives have passed? 12.5% 15% 25% 60%?

12.5%

Explanation:

The half life of any reaction is the amount of time it takes to consume half of the starting material.

In this case, the original or starting percentage of material is 100%

100% - 50% : First Half Life

50% - 25% : Second Half life

25% - 12.5% : Third Half life.

After three half lives have passes, the percentage of strontium-90 that remains in a sample is 12.5%

12.5% of strontium-90 would remain in a sample after three half-lives have passed. Half-life automatically means 50% of the original amount would remain.