[填空题]
A typical banana contains 400 mg of potassiumf kae/ l.; :jigo*z+7ygur w6e, of which a small fraction is thcu u+m4k jk/2u(w) spae radioactis+(c4k wauumup / jk)2ve isotope $_{19}^{40}\text{K}$ (see Appendix B). Estimate the activity of an average banana due to $_{19}^{40}\text{K}$. $\lambda N$ = $\ \text{decays/s}$
