text: ## Elementary Statistics (12th Edition)
  
  a)We know that $probability=\frac{number \ of \ good \ outcomes}{number\ of\ all\ outcomes}$, hence $P(Fraternal \ twins)=\frac{5+5+5+5}{5+0+0+5+5+5+5+5}=0.666.$ b)We know that $probability=\frac{number \ of \ good \ outcomes}{number\ of\ all\ outcomes}$, hence $P(Fraternal \ twins|Boy \ and \ Girl)=\frac{5+5}{0+0+5+5}=1.$