a) 2087 + ( X + 25 ) = 26 + 2087
2087 + ( X + 25 ) - 2087 = 26
( 2087 - 2087 ) + ( X + 25 ) = 26
0 + ( X + 25 ) = 26
X + 25 = 26
X = 26 - 25
X = 1
b) X + 4321 + 2165 = 2155 + 4311 + 20 + 21
X + 4321 + 2165 = 2155 + 4311 + ( 10 + 10 ) + 21
X + 4321 + 2165 = ( 2155 + 10 ) + ( 4311 + 10 ) + 21
X + 4321 + 2165 = 2165 + 4321 + 21
X + (4321 + 2165) - (4321 + 2165) = 21
X + 0 = 21
X = 21 - 0
X = 21
c) ( X + 2 ) x 1996 = 1996 x 3
=> X + 2 = 3
X = 3 - 2
X = 1
25 x X < 25 x 4
25 = 25 nên X < 4 để 25 x X < 25 x 4
X < 4 => X = 0 ; 1 ; 2 ; 3
Xong rồi đó nha ~~