The half life of radium is about 1600 years. Of 100g of radium existing now, 25g will remain undecayed after :-

Initial mass $M_0 = 100$ g; remaining (active) mass $M = 25$ g.
$\dfrac{M_0}{M} = \dfrac{100}{25} = 4 = 2^2$
Since $M = M_0\left(\dfrac{1}{2}\right)^n$, the number of half lives $n = 2$.
Therefore required time $= 2 \times T_{1/2} = 2 \times 1600 = 3200$ years.
(100 g $\rightarrow$ 50 g after 1600 years $\rightarrow$ 25 g after 3200 years.)