Uranium isotope relative abundance at diff times

[thiev]

Homework Statement

The relative abundance of uranium isotopes in today’s Earth crustal material is:
99.2745% 238U
0.7196% 235U
0.0055% 234U
What were the relative abundances when the Earth was formed 4.6x109 years ago?

Homework Equations

t = age = 1/lambda * ln(N/No)

N = No*e^lambda*t

The Attempt at a Solution

i don't know how to approch this...

 

[LowlyPion]

Welcome to PF.

What are the half-lives of the 3 isotopes?

Since they all decay to Thorium then maybe if you work them backwards you can turn back the clock to see what was around 4 and a half billion years ago?