18) B
Total number of parts = 7 + 1 = 8
The maximum age would be when the rock contained 8 parts of ²³⁵U.
Here are the half lives:
| Half – life | ²³⁵U | ²⁰⁷Pb |
| 0 | 8 | 0 |
| 1 | 4 | 4 |
| 2 | 2 | 6 |
| 3 | 1 | 7 |
So it was after 3 half-lives that the ratio became 1 : 7
Half-life = 7.1 x 10⁸ years
So maximum age of rock = 3 x 7.1 x 10⁸ = 21.3 x 10⁸ = 2.13 x 10⁹ years