Hi Davie T.,
100(1.05)2t = 500,000
1.052t = 5000...........................................[divide both sides by 100]
ln(1.052t) = ln(5000)................................[take the natural log of both sides]
2t*ln(1.05) = ln(5000)..............................[ln(ab) = b*ln(a)]
2t = ln(5000)/ln(1.05)...............................[divide both sides by ln(1.05)]
t = ln(5000)/ln(1.052) ≈ 87.2839..............[divide both sides by 2, b*ln(a) = ln(ab)]
I hope this helps, Joe.