A = (20+21+22)+23( 20+21+22+23)+27( 20+21+22+23)+....................+22004(20+21+22+23)+22007(20+21+22+23)
= 7 + 15.23 + 15.27 + .......................+ 15.22004 + 15.22007
= 7 + 15.(23 + 27 + .....................+ 22004 + 22007)
A chia cho 15 dư 7