Question 81809
the half-life equation is correct ... you need the fact that k=(ln2)/h, h=(half-life) and t=(elapsed time)


it would be easier to see (and remember) if you use y=y0*(1/2)^(t/h)


dividing by y0 and taking the natural log of both sides gives ln(y/y0)=(ln(1/2))(t/h)


dividing by ln(1/2) and multiplying by h gives h*ln(y/y0)/ln(1/2)=t ... so t=2.47 billion years