So we are required to calculate the half life of U-235 after 2100 million years.
We are given that a half life is 700 million. Therefore we want to know the fraction of a sample after 3 half lives.
We are not given the amount of the sample before and after decay.
If you are not given amounts start at 100%. So initially you have 100% after 700 million years (first half life) you are left with 50%, after another 700 million years (second half life) you are left with 25% and after another 700 million years (third half life) you are left with 12.5% (12.5/100).
12.5% = 0.125 = 1/8