Answer:
683 million years
Explanation:
For radioactive nuclides, the time to decay is given by the expression:
t = -2.303 /k log (A/A₀)
where k is the decay constant and is calculated from the equation:
k = 0.693/ t₁/₂ where ₁/₂ is the half-life.
k = 0693/ 703 x 10⁶ yrs = 9.85 x 10⁻¹⁰ /yr
A = amount after time t
A₀ = amount originally present
Then
t = -2.303 / 9.85 x 10⁻¹⁰ /yr x log (0.51) = 683 x 10⁶ yr
