Half-life of a rock = 1.25 billion years
Rock lives of the rock found = 3.6 half-lives
Multiply the number of billion years of each half live (1.25) by the number of half-lives of the rock found:
1.25 x 3.6 = 4.5 billion years
Since the solar system started 4.6 billion years ago, the rock found is not from the start of the solar system.
