Strontium-90 rained down on northern Scandinavia in 1986. What year will northern Scandinavia be 100% free from radiation?

Strontium 90 has a half life of about 29 years. The way that half life works is that the concentration of the radioactive strontium is cut in half every 29 years. If you start with a concentration of 'x', then after 29 years the conentration is 1/2 x. After an additional 29 years the concentration is cut in half again, so you have 1/4 x. And this pattern continues every 29 years, going to x/8, then x/16, etc. Hence the concentration after 'n' half-life periods is x/(2^n). So to answer this question - you need to figure out at what point does x/(2^n) = 0?