a)(-2/5).4/15+(-3/10).4/15= 4/15. [ (-2/5) + (-3/10) ]
= 4/15. (-4/10) + (-3/10)
= 4/15 . (-7/10)
= -14/75
b,21-3 3/4:(3/8-1/6) = 21 - 15/4 : ( 9/24 - 4/24 )
= 21 - (15/4 : 5/24)
= 21 - ( 15/4 . 24/5)
= 21 - 360/20
= 21 - 18
= 3
c)(-5/6+2/5):3/8+(4/5-11/30):3/8 = (-5/6+2/5).8/3 + (4/5-11/30).8/3
= 8/3. [(-5/6)+2/5 + 4/5-11/30 ]
= 8/3. [(-25/30) - 11/30] + (2/5 + 4/5)
= 8/3. [(-36/30) + 6/5]
= 8/3. [( -6/5 )+ 6/5 ]
= 8/3. 0
= 0