Answer
$77.22\%$
Work Step by Step
Number of moles of barium sulfate:
$0.2070\ g\div 233.38\ g/mol=0.8869\ mmol$
From stoichiometry, same number of moles of saccharin:
$0.8869\ mmol\times 183.18\ mg/mmol=162.5\ mg$
Mass percent:
$162.5/210.4\times 100\%=77.22\%$
