a, (-0,35) . 3/14 . (-3 5/7) . (-4/21)
= (-7/20) . 3/14 . (-26)/7 . (-4/21)
= (-3/40) . (-26)/7 . (-4)/21
= 13/140 . (-4)/21
= (-13)/735
b, (-3)/7 . 5/11 + (-5)/14 . 5/11
= (-6)/14 . 5/11 + (-5)/14 . 5/11
= [(-6)/14 + (-5)/14] . 5/11
= (-11)/14 . 5/11
= (-5)/14
c, 15 - 2 1/2 : (4/9 - 1/6)
= 15 - 5/2 : (8/18 - 3/18)
= 15 - 5/2 : 5/18
= 15 - 5/2 . 18/5
= 15 - 9
= 6
d, (-3/4 + 2/5) : 3/7 + (3/5 + -1/4) : 3/7
= [(-3/4) + 2/5 + 3/5 + (-1)/4] : 3/7
= {[(-3/4) + (-1/4)] + [2/5 + 3/5]} . 7/3
= {(-1) + 1} . 7/3
= 0 . 7/3
= 0