Question

PLEASE HELP!

20 grams of a radioactive substance has a half life of 30 days

how much will be left after 150 days?

-1.25g

-2.50g

-0.63g

-0.31g

Answer 1

Answer: 0,63 g

Explanation: During 150 days 5 decays happen. You can write a set

of masses : 20 g ⇒ 10 g after first decay, then 5 g , 2.5 g , 1,25 g

and after fifth decay 0.63 g

Answer 2

150/30 = 5
HF1 20/2 = 10
HF2 10/2 = 5
HF3 5/2 = 2.5
HF4 2.5/2 = 1.25
HF5 1.25/2 = 0.625
Answer: 0.63g