science
posted by tanya .
In the laboratory small zircon crystals were separated from granite (K).
Isotopic analysis of these zircon crystals gives a 207Pb/235U ratio of 0.45. the halflife of 235U (7.04 × 108
years), estimate the age of the granite in Ma to two significant figures

I would start by turning this thing around so that (235U/207Pb) = 1/0.45 = 2.22
Then I would adjust the ratio to account for the fact that Pb has a lower atomic mass than U by a factor of 235/207 = 1.135 so 2.222/1.135 = 1.958.
For every x atoms U that decay to Pb, the Pb will be x and the U remaining will be 100x; therefore,
[(100x)/x] = 1.1958
Solve for x = (Pb) and (100x) U.
Then (No/N) = kt
for No = 100
For N = what you found above for 100x
k see below.
t = time.
k = 0.693/t_{1/2</sub. k = 0.793/7.04E8} 
I'm going out on a limb here, and I know you might not reply, but, DrBob222, I need to ask you something concerning this.
Why did you flip the ratio? Wouldn't that effect the answer?
And is that a mistake in the middle where you said,
"Then I would adjust the ratio to account for the fact that Pb has a lower atomic mass than U by a factor of 235/207(!that is what I'm talking about!) = 1.135 so 2.222/1.135 = 1.958."
Shouldn't that be flipped? I ran it through a calculator and the flipped version gave '1.135' like you said the other version equalled here.