# The half-life of calcium-47 about 5 days. Starting with 64 g of this isotope, what would be the amount remaining after 20 days?

Aug 3, 2016

The answer is 4 g.

#### Explanation:

Divide 64 by 2 four times, because there are four half-lives in 20 days.

Every half-life, the calcium-47 will lose half of the amount remaining.

$\text{20 days"/"5 days/half-life" = "4 half-lives}$.

$\text{64 g"/2 = "32 g}$ after 1 half-life

$\text{32 g"/2 = "16 g}$ after 2 half-lives

$\text{16 g"/2 = "8 g}$ after 3 half-lives

$\text{8 g"/2 = "4 g}$ after 4 half-lives.