Answer:
60 hours
Explanation:
We are given;
Initial amount: A_o = 64 mg
Half life; T_½ = 15 hours
Amount left: A = 4 mg
To get the time that 4 mg of Sodium-24 remain, we will use the formula;
A = A_o(½^(t/T_½))
Where t is the time that 4 mg of Sodium-24 remain.
Thus;
4 = 64 × ½^(t/15)
Divide both sides by 64 to get;
4/64 = 2^(t/15)
0.0625 = ½^(t/15)
In 0.0625 = (t/15) In 0.5
-2.7726 = (t/15) × -0.6931
t = (-2.7726 × 15)/-0.6931
t = 60 hours