$1)$ Half$-$life of an element is t second. Calculate the percentage left undecayed in t$/2$ sec$.$

$2)54\mathrm{.}5$ mg Na$3$PO$4$ contains $32^$P $(15\mathrm{.}6\%)$ and $31^$p atoms. Assuming only $32^$p atoms are radioactive, calculate the rate of decay for the given sample of Na$3$ PO$.$ The half$-$life period of $32^$P$=14\mathrm{.}3$ days; mol. wt$.$ of Na$3$PO$4=161\mathrm{.}2.$