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

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

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

Try b? sorry if i’m incorrect