Answer:
30
Step-by-step explanation:
First, find out how many half-lives have passed. Divide the number of minutes by the length of a half-life.
20÷10=2
2 half-lives have passed.
Now figure out how much nitrogen will be left after 2 half-lives have passed. Multiply the starting amount of nitrogen by
1/2
a total of 2 times.
120x1/2 1/2=30
That calculation could also be written with exponents:
120x(1/2)2=30
After 2 half-lives, the amount of nitrogen left will be 30 grams.