Suppose 128 radioactive atoms have a half-life of 15 seconds. Approximately how many radioactive atoms will be left after 60 seconds?

A. 32

B. 16

C. 8

D. None

no you cannot do 128/15. that wouldn't give you 8 in any case. but your answer is right.

Half-life = 15 seconds.

Start with 128 atoms.

Each atom has a 50% chance of falling apart within 15 seconds.

For such a small group of atoms, we have to assume that exactly

half of the atoms die every 15 seconds.

Number of atoms at the beginning of the time = 128

Number of atoms after 15 sec (one half life) = 64

Number of atoms after 30 sec (another half life) = 32

Number of atoms after 45 sec (another half-life) = 16

Number of atoms after 60 sec (another half-life) = 8