help

$p=0.001\\ a)~P[\text{no one in 10000 has blood type}]=\left(1-0.001\right)^{10000} = \\ (0.999)^{10000} = 0.0000451733\\~\\ b)~P[\text{at least 1 w/type in $n$ tested}] = \\ 1-P[\text{none w/type in $n$ tested}]=\\ 1-(1-0.001)^n = 1-(0.999)^n \geq \dfrac 1 2\\ \dfrac 1 2 \geq (0.999)^n\\ \ln(1/2)\geq n \ln(0.999)\\ \dfrac{\ln(0.5)}{\ln(0.999)} \leq n\\ n \geq 693$