Respuesta :
'of' can be translated into multiply so
9/10 of 90=9/10 times 90
90=90/1
9/10 times 90/1=(9 times 90)/10=810/10=81/1 times 10/10=81
9/10 of 90=9/10 times 90
90=90/1
9/10 times 90/1=(9 times 90)/10=810/10=81/1 times 10/10=81