Krypton-85 has a half-life of 10.76 years. It decays into rubidium-85. Suppose a sample contains 4 atoms of krypton-85 and 12 atoms of rubidium-85. About how old is the sample?

However, as of 2009 the total amount in the atmosphere is estimated at 5500 PBq due to anthropogenic sources.[5] At the end of the year 2000, it was estimated to be 4800 PBq,[4] and in 1973, an estimated 1961 PBq (53 megacuries).[6] The most important of these human sources is nuclear fuel reprocessing.[4][5][6] Nuclear fission produces about three atoms of krypton-85 for every 1000 fissions; i.e. it has a fission yield of 0.3%.[7] Most or all of this krypton-85 is retained in the spent nuclear fuel rods; spent fuel on discharge from a reactor contains between 0.13-1.8 PBq/Mg of krypton-85.[4] Some of this spent fuel is reprocessed. Current nuclear reprocessing releases the gaseous 85Kr into the atmosphere when the spent fuel is dissolved. It would be possible in principle to capture and store this krypton gas as nuclear waste or for use. The cumulative global amount of krypton-85 released from reprocessing activity has been estimated as 10,600 PBq as of 2000.[4] The global inventory noted above is smaller than this amount due to radioactive decay; a smaller fraction is dissolved into the deep oceans.[4]

it is the second option.