( X + 9 ) + ( X - 8 ) + ( X + 7 ) + ( X - 6 ) + ( X + 5 ) + ( X - 4 ) + ( X + 3 ) = 90
X + 9 + X - 8 + X + 7 + X - 6 + X + 5 + X - 4 + X + 3 = 90
( X + X + X + X + X + X + X ) + ( 9 - 8 + 7 - 6 + 5 - 4 + 3 ) = 90
7 × X + 6 = 90
7 × X = 90 - 6
7 × X = 84
X = 84 : 7
X = 12
Chúc bạn học tốt ^^