[bigbaz]

The composition of natural uranium at the present time is approx 99.2 % U238 and 0.711 % U235. However U235 has a half life of about 700 million years and U238 about 4500 million years so working back from now to then the U235 % doubles 6.5 times while the U238 % doubles once. This means that the % split of natural uranium when the earth was formed was about 70% U238 and 30% U235.
Is this correct ? Can anyone see a flaw in this deduction?

[FeLiXe]

it looks good. but if you are really wondering about this (and it's not a test question), you have to ask if 235U was formed by the decay of some other nuclide