Answer:
source A 50%
source B 25%
Step-by-step explanation:
Half life is time it takes for something to get reduced by half. For example, if half life of my weight is 5 years and I am currently 20lbs, then in next 5 years I will become half of my original so 10lbs.
Similarly, if compound A has half-life of 10 million years, then after 10 million years, half of compound A will remain, thus 50% remains
Compound b has half life of 5 million years, so first 5 million years it will reduce by half so become 50% but then in another 5 million years (total of 10 million years) it gets reduced by another half so 50/2 = 25% remains.