The carbon in living organisms contains about $1.3$ atoms of ${ }^{14} \mathrm{C}$ for every $10^{12}$ atoms of all kinds of carbon, and the half-life of ${ }^{14} \mathrm{C}$ is 5730 years. (a) How many decays would one expect per minute from $7 \mathrm{~g}$ of carbon taken from a living tree? (b) If $7 \mathrm{~g}$ of carbon is extracted from a wooden beam of a prehistoric dwelling and if they undergo 10 decays per minute, about how old is the dwelling?