# Question #24d75

Apr 25, 2017

7.5 grams.

#### Explanation:

Divide 12 hours by the half life of 3 hours to find the number of half lives.

$\frac{12}{3} = 4$ = the number of half lives.

A half life divides the amount by half so to find the mass left divide by ${2}^{4}$ or $2 \times 2 \times 2 \times 2 = 16$

so divide 120 grams by 16 or ${2}^{4}$

$\frac{120}{16} = 7.5$ grams.