a. ( 72 - 8 x9) : ( 20 + 21 + 22 + 23 + 24 + 25)
= ( 72-72) : ( 20 + 21 + 22 + 23 + 24 + 25)
= 0 : ( 20 + 21 + 22 + 23 + 24 + 25)
= 0
b, ( 500 x 9 - 250 x 18 ) x ( 1 + 2 + 3 + ...+ 9)
= ( 250 x 2 x 9 - 250 x 18 ) x ( 1 + 2 + 3 + ...+ 9)
= ( 250 x 18 - 250 x 18 ) x ( 1 + 2 + 3 + ...+ 9)
= 0 x ( 1 + 2 + 3 + ...+ 9)
= 0
c, (11 + 13 + 15 + ...+ 19) x ( 6 x 8 - 48)
= (11 + 13 + 15 + ...+ 19) x ( 48 - 48)
= (11 + 13 + 15 + ...+ 19) x 0
= 0