100 - [ 250 - 5 × ( x - 5 ) : 5 ] × 3 = 40
[ 250 - 5 × ( x - 5 ) : 5 ] × 3 = 100 - 40
[ 250 - 5 × ( x - 5 ) : 5 ] × 3 = 60
250 - 5 × ( x - 5 ) : 5 = 60 : 3
250 - 5 × ( x - 5 ) : 5 = 20
5 × ( x - 5 ) : 5 = 250 -20
5 × ( x - 5 ) : 5 = 230
5 × ( x - 5 ) = 230 × 5
5 × ( x - 5 ) = 1 150
x - 5 = 1 150 : 5
x - 5 = 230
x = 230 + 5
x= 235