Answer
$3,000$ years
Work Step by Step
When $ 30\%$ of $A$ has decayed, the amount that remains is $0.70A$.
We want the $t$ for which
$0.70A=A(0.999879)^{t}$
$0.70=(0.999879)^{t}$
$\log 0.70=t\cdot\log 0.999879$
$t=\displaystyle \frac{\log 0.70}{\log 0.999879}\approx 2947.548$
or, about $3,000$ years