If an isotope has a half-life of 50 million years and an 1/8 of the sample remaining consists of that isotope, the age of the rock from which it was taken is:

150 million years old

Explanation:

If we have an isotope that has a half-life of 50 million years, we need just need to divide the numbers in order to get to one eight of the full number, than multiply it by 50 million in order to get the result.

If one half is 50 million years, than dividing one more half, which will gives one fourth of the total, will brings us to 100 million. Dividing the one fourth by two will leads to the one eight of the full number, and adding 50 million more years, we get 150 million years. So the rock in question has 150 million years of age.