6. 90-(15+9): 8 18+9*(13-7) 64-(28+4): 4 72:9+2*7 9*6-30:3 28:7+5*6 []*8=56 6*[]=54 []*9=63

90 - (15 + 9) : 8 = 90 - 24 : 8 = 90 - 3 = 87 18 + 9 * (13 - 7) = 18 + 9 * 6 = 18 + 54 = 72 64 - (28 + 4) : 4 = 64 - 32 : 4 = 64 - 8 = 56 72 : 9 + 2 * 7 = 8 + 14 = 22 9 * 6 - 30 : 3 = 54 - 10 = 44 28 : 7 + 5 * 6 = 4 + 30 = 34 [] * 8 = 56 => 7 * 8 = 56, [] = 7 6 * [] = 54 => 6 * 9 = 54, [] = 9 [] * 9 = 63 => 7 * 9 = 63, [] = 7