Question

A radioactive kind of nitrogen has a half-life of 10 minutes. If you start with 64 grams of the substance, how much will be left after 20 minutes

Answer 1

30

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.

Answer 2

There would be 16 grams left after 20 minutes.
64/2 = after ten minutes
= 32
32/2 = after another ten minutes (20 full)
= 16