# Question #e627c

We can use the direct formula for this, or use a simple arithmetic ratio if the amounts work out well. The definition of half-life is the time it takes for half of the substance to decay. Thus, after one (1) half-life, $\frac{1}{2}$ of the sample is left. After another half-life, half of THAT will be gone: $\frac{1}{2} \cdot \frac{1}{2} = \frac{1}{4}$.
Putting them together we conveniently get our desired $\frac{3}{4}$ decayed thorium.