Đề bài

Tìm x, biết:

a) $- 3x + 7 = 12 - 125$

b) $\frac{1}{3}:\left( {2x - 1} \right) = \frac{{ - 4}}{{21}}$

c) $\left[ {124 - \left( {20 - 4x} \right)} \right]:20 = 12$

d) $\left( {\frac{1}{{2.3}}\, + \,\frac{1}{{3.4}} + ... + \frac{1}{{8.9}} + \,\frac{1}{{9.10}}} \right)\,.\,x = \frac{1}{5}\,$