a) -2/7. 21/8 = -2 . 21/7.8
= -2.3.7/7.2.4
= -3/4
b) 0,24. (-15/4) = 6/25. (-15/4)
= 6.(-15) / 25.4
= 2.3.(-5).3 / 5.5.2.2
= 3.(-3) / 5.2
= -9 /10
c)(-2).(-7/12) = -2.(-7) / 12
= 2.7 / 2.6
= 7/6
d) (-3/25):6 = -3/25.1/6
= -3.1 / 25.6
= -3/25.2.3
= -1/25.2