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## Homework Statement

The Age of the Earth is 4.5 × 109 y. Use the information given in the following table to

calculate the ratio of the number of 238U nuclei at the present time to the number of 235U nuclei

at the time of the formation of the Earth.

Isotope Half-life Current natural abundance(%)

238U 4.468 ×109 y 99.274

235U 7.04 ×108 y 0.72

## Homework Equations

I used T1/2= ln(2)/lamda to work out lambda for both 235 and 238Uranium.

I converted time in years to seconds by no. of years * 3.156x10^7 ( I saw it done in my book don't really understand it but anyway :) )

I used N(t)=N0*e^-lambda*t to work out the number of radioactive nuclei at a given time.

## The Attempt at a Solution

So basically just using those two equations I came to

238U N(t) and present to be = 0.498N0

and 235UN(t) at beginning, (hence t=0) to be N0,

so i said the ratio is 0.498... However upon doing this I realized I didnt take into account the last column of the table given in the quetsion, and seeing as the question says taking into account the following information, I figure I have gone about this the wrong way...

Any help much appreciated :)