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: \(\frac{1}{2^n} * A_0 = A\) \(\frac{1}{2^n} * 120 = 15\) \(\frac{1}{2^n}\) = \(\frac{15}{120}\) \(\frac{1}{2^n}\) = \(\frac{1}{8}\) \(\frac{1}{2^n}\) = \(\frac{1}{2^3}\) \(2^n = 2^3\) n = 3 :6days \div 3 = 2days