Đề bài

Tính

a) ${{ - 3} \over 5}.{{15} \over { - 7}}.{{21} \over 6}$                       

b) ${{ - 33} \over {25}}.{{15} \over {11}}.{{21} \over 6}$

c) ${{ - 22} \over {25}}:{{11} \over 5}.{{21} \over 6}$                       

d) ${{ - 3} \over 5}.\left[ {\left( { - {{15} \over {11}}} \right) - {{21} \over {33}}} \right]$