Answer:
3.125 grams
Explanation:
Applying,
R = ()R'............... Equation 1
Where R = Original mass of Astatine-211, R' = mass of Astatine-211 left after decay, t = total disintegeration time, n = halflife of Astatine-211
make R' the subject of the equation
R' = R/()............ Equation 2
From the question,
Given: R = 50 grams, t = 28 hours, n = 7 hours
Substitute into equation 2
R' = 50/()
R' = 50/(2⁴)
R' = 50/16
R' = 3.125 grams