$a) x\times2009-x=2009\times2007+2009$
$x \times (2009-1) = 2009 \times (2007 + 1)$
$x \times 2018 = 2009 \times 2018$
$x = 2009$
Vậy $x=2009$
$b) (x+1)+(x+2)+(x+3)+(x+4)+(x+5)=45$
$ 5 \times x + (1+2+3+4+5) = 45$
$5 \times x + 15 = 45$
$5 \times x = 30$
$x =6$
Vậy $x=6$
$c) (x\times0,25+1999)\times2000=(53+1999)\times2000$
$x \times 0,25 + 1999 = 53 + 1999$
$x \times 0,25 = 53$
$x = 53: 0,25$
$x = 212$
Vậy $x=212$.