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%

answer : one can design an experiment using a liquid and an additive that can be added to the liquid and observe the chemical changes in the reaction species.

one would need a beaker which is filled with a known amount of the liquid in which thermometer is placed to measure the temperature changes; after addition of the additive to the liquid solution one can note down the temperature change and find whether the reaction is endothermic or exothermic in nature. by subtracting the initial temperature value with the final temperature one can find the temperature changes.

various combinations of liquid and additive can be used. liquid in this case can be a known quantity of water and additives can be epsom salt or without epsom salt the experiment can be conducted to notice the changes. water and ice, hydrogen peroxide and dry yeast, vinegar and baking soda,etc.