Naturally occurring uranium is a mixture of 238u (99.28%) and 235u (0.72%) isotopes. i) how old must the material of the solar system must be if one assumes that at its creation both isotopes were present in equal quantities? how do you interpret this result? the age of the planet earth is estimated at 4.5 billion years. the lifetime of 235u is t235 = 1.015 ~ 109 years. the lifetime of 238u is t238=6.49 ~ 109 years. ii) obtain the fraction of the initial 238u amount that has decayed since the formation of the earth's crust 2.5 x 10'years ago?