Age of rocks calculation - I only need little help

How many half lives have passed since a particular rock was formed if a sample of the rock contains 31 times as much lead-207 than uranium-235?

I know that I need to do a table and calculate

I know how to do this question but the thing I need help on is how do I know the number of atoms in order to do a table and calculate. Like I have the ratio of parents:daughter isotope which is 1:31 right? but how do I do this? Just tell me how and I will do it myself. Actually I don't know physics, I'm studying it all by myself and I got this question from my friend's homework and I'm stuck on it, there are 3 questions like it and I need help on how to turn this ratio into actual numbers in order to calculate how many half lives have passed. If I don't need to turn it into numbers then please tell me how do I solve this question? Thank you :)

EDIT: okay so thats what I did is this correct?

1 - 31 half lives=5
2 - 30 half lives=4
4 - 28 half lives=3
8 - 24 half lives=2
16 - 16 half lives=1
32 - 0 half lives=0
so 5 half lives have passed
Correct??

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ehild
Homework Helper
5 half lives is correct.

ehild

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Hmm? You didn't really explain anything...how did you get to 32? I got to 5.

ehild
Homework Helper
I meant 5, sorry... As the original amount decreased to 1/32-th.

hild

oo thank you :)..and my way is correct do I have to do any formulas?
Thank you very much ^^

ehild
Homework Helper
The daughter atoms present + number of un-decayed U235 atoms=N(0):
31N + N =N(0)

The number of un-decayed isotopes present at time t is

N(t)=N(0) (1/2)t/halflife,

where N(0) is the number initially.

N=N(0)/32, 32 =25, ....

ehild

I see :D Now I understand everything, thank you!