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.

Electric force between two charged object varies inversely as the square of distance between them (coulomb's law). this means if two charged objects move away force between them decreases.