Question

If you have 10.0 g of a substance that decays with a half-life of 14 days, then how much will you have after 42 days?

Answer 1

The formula for the mass that remains:
$m=m_0 \times (\frac{1}{2})^\frac{t}{T}$
m₀ - the initial mass, t - time, T - the half-life

$m_0=10 \ g \\ T=14 \ d \\ t=42 \ d \\ \\ m=10 \times (\frac{1}{2})^\frac{42}{14}=10 \times (\frac{1}{2})^3=10 \times \frac{1}{8}=10 \times 0.125=1.25$

The answer is c. 1.25 g.