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 => 56 / 8 = 7
6 * [] = 54 => 54 / 6 = 9
[] * 9 = 63 => 63 / 9 = 7