[填空题]
Calculate the mass of a samb vl542+* tun;cyhyb4 rk;pou kmth8,ple gx0s* 9-)e pov *5czwpnwthd2of pure $_{19}^{40}\text{K}$ with an initial decay rate of $2.0 \times 10^5\ \text{s}^{-1}$. The half-life of $_{19}^{40}\text{K}$ is $1.28 \times 10^9\ \text{yr}$. m = $g$
