The starting amount of radioactive iodine-131 was 250 g. The half-life is 8 days. When Leslie measured the amount, she measured 15.625 g of radioactive iodine. How many days have elapsed between measurements?

A. 16

B. 24

C. 32

D. 64

Divide the 250g in half. That is 125g.

Next divide the 125g in half. That is 62.5g.

Divide 62.5g in half. That is 31.5g.

Divide the 31.5g in half. That is 15.625g

Next count how many half-life splits there were. There were 4.

Then multiply the 4 half-life slips by 8 for the amount of days in he half-life.

4x8=32

32 days to reach 15.625g. The answer is C.