chemistry
posted by lolo .
How long will it take for the 100mg of cesium137 to decay to 6.25mg(1/16 the original amount)?

2^{n} = 16
Solve for n = number of half lives. 
still i can't understand it

Do you have the halflife given in the problem or are you allowed to look it up? If so there is an easier way to set it up.
To solve 2^{n} = 16, take the log of both sides.
log 2^{n} = log 16
n*log 2 = log 16
n*0.301 = 1.2 etc.
There is another way. Remember that halflife means the time it takes for 1/2 the material to disappear. So make a table like this. The periods are used for spacing.
amount......# halflives
100 mg.........0
50 mg..........1
25 mg..........2
12.5 mg........3
6.25 mg........4
So whatever the halflife is, multiply that by 4 and you will have the time it takes to go through 4 halflives. 
thanks alot , this answer really helped me to understand this question