<h3>
Answer:</h3>
20 g
<h3>
Explanation:</h3>
We are given;
Original mass of K-40 before decay = 80 g
Half life of Potassium-40 = 1.25 billion years
Time taken by the decay = 2.5 billion years
We are required to calculate the mass of K-10 that will remain after 2.5 billion years.
Using the formula;
Remaining mass = Original mass × (1/2)^n
where n is the number of half lives
Number of half lives, n = Time taken ÷ half-life
= 2.5 billion years ÷ 1.25 billion years
= 2
Remaining mass = 80 g × (0.5)^2
= 80 g × 0.25
= 20 g
Therefore, the mass of K-40 isotope that will remain after 2.5 billion years is 20 g.
