Answer :
[tex]x-number\ of\ child's\ plates\\250-x-number\ of\ adult's\ plates\\\\5.5x+9(250-x)=1935\\5.5x+2250-9x=1935\\5.5x-9x=1935-2250\\3.5x=315\ \ \ \ \ \ |:3.5\\x=90\\\\250-90=160\\\\There\ where\ 90\ child's\ plate\ and\ 160\ adult's\ plate.[/tex]