Question

Question #e627c

Answer 1

37.44 days

Explanation:

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, `1/2` of the sample is left. After another half-life, half of THAT will be gone: `1/2 * 1/2 = 1/4`.
Putting them together we conveniently get our desired `3/4` decayed thorium.
With a half-life of 18.72 days, this amount of decay will occur in 37.44 days.