The equation that establishes a relationship between the amount left undecayed,
A, the initial amount, A0, and the number of half-lives that pass in a period of time t looks like this:
12n*A0=A
12n*120=15
12n = 15120
12n = 18
12n = 123
2n=23
n = 3 :6days ÷ 3 = 2days
