# The half life of thorium-227 is 18.72 days. How many days are required for 75% of a given amount to decay?

Nov 11, 2016

#### Answer:

If 75% has decayed, 25% or 1/4 is left.

#### Explanation:

1/4 is a half of one half, so it will take two half-lives.
$2 \times 18.72 = 37.44$ days.