The mass of a radioactive element at time t is given by
[tex]m(t) = m_0 ( \frac{1}{2} )^{ \frac{t}{t_{1/2}} }[/tex]
where [tex]m_0[/tex] is the mass at time zero, while [tex]t_{1/2}[/tex] is the half-life of the element.
In our problem, [tex]m(t)=2.64 g[/tex], t=121.0 s and [tex]t_{1/2}=60.5 s[/tex], so we can find the initial mass [tex]m_0[/tex]:
[tex]m_0= \frac{m(t)}{ (\frac{1}{2})^{t/t_{1/2}} } = \frac{2.64 g}{( \frac{1}{2} )^{121/60.5}} =4 \cdot 2.64 g=10.56 g[/tex]
