a/ 300 - 5( x + 2 ) = 100
5( x + 2 ) = 300 - 100 = 200
x + 2 = 200 : 5 = 40
x = 40 - 2 = 38
b/ ( 5x - 10 ) . 10 = 1000
5x - 10 = 1000 : 10 = 100
5x = 100 - 10 = 90
x = 90 : 5 =18
c/ 800 + 2( x + 7 ) = 1200
2( x + 7 ) = 1200 - 800
2( x + 7 ) = 400
x + 7 = 400 : 2 = 200
x = 200 - 7 = 193
d/ ( 10x + 100 ) + 300 = 800
10x + 100 = 800 - 300 = 500
10x = 500 - 100 = 400
x = 400 : 10 = 40