Radioactive decay half-life of polonium

[pat666]

Homework Statement

c) In 420 days, the activity of a sample of polonium, Po, fell to one-eighth of its initial value. i) Calculate the half-life of polonium.

Homework Equations

The Attempt at a Solution

if the activity fell by 1/8 then it went through 4 half liefs (1/2*4=1/8) so the half life is simply 420/4=105days. The wording confuses me though because of the use of "activity" so I think what I've done might be wrong?
Thanks

 

[NobodySpecial]

Half life is the time for the activity to half.
How many times do you have to cut something in half to get 1/8 of the original?

 

[pat666]

isnt it 4 like i said? (1/8*4=1/2)
Thanks

 

[NobodySpecial]

1 * 1/2 = 1/2
1/2 * 1/2 = 1/4
1/2 * 1/2 * 1/2 = 1/8

 

[pat666]

oops, I don't know what I was thinking. so t1/2 would be 140days?

 

[NobodySpecial]

correct

 

[pat666]

sweet, thanks