p = 100-10 = 90%
p = m pur/m impur×100 => m pur = p×mimp/100 = 90×100/100 = 90g Al2O3
MAl2O3 = 2AAl + 3AO = 2×27+3×16 = 102g/mol
102g Al2O3.....................2×27g Al
90g Al2O3............................xg Al
x = 54×90/102 = 47,647g Al
Good luck !