Answer:
more than 75%
Step-by-step explanation:
***Pay attention to the wording***
A potassium isotope has a half-life of 1.25 billion years. Given that scientists estimate Earth's age to be 4.6 billion years, what is the most likely percentage of parent to daughter isotopes of this element currently existing on Earth?
Since they are talking about daughter isotopes there would be more than 75% left since the potassium isotope has a half-life of 1.25 billion years and it has been approximately 4.6 billion years!
However, if it was asking about parent isotopes, the answer would be less than 10%.
In this question, and many others, it is important to pay attention to the wording!
Have a sublime day!