750 million years ago, there was enough Uranium-235 present in Oklo, Gabon to have a natural fission reactor occur and generate a chain reaction. If there is currently 50kg of U-235 present in Oklo, how much must have been present 750 million years ago when the reaction took place?

With a half-life of 700 million years, U-235 would have had twice as much mass at a time 700 MYA. This would have put the mass at 100kg at that time. Going back another 50 million years would be (50/700) or 1/14 of the half-life, or (1/2 * 1/14), or 1/28 of the total mass. 1/28 of 100kg is 3.57kg, so the amount present at the 750MYA mark would be approximately 103.57kg of U-235.