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 ÷ 3 = 2days