câu a là đáp án C, câu b là đáp án B

c

b

x + 20 + 21 + x + 22 + 23 + x + 24 + 25 + x + 26 + 27 + x + 28 + 29 + x + 30 = 330

6x + (30 + 20) . (30 - 20 + 1) : 2 = 330

6x + 50 . 11 : 2 = 330

6x + 275 = 330

6x = 330 - 275

6x = 55

x = 55 : 6

x = 55/6

\(x+20+21+x+22+23+x+24+25+x+26+27+x+28+29+x+30=330\)

\(\Rightarrow\left(x+x+x+x+x+x\right)+\left(20+21+22+23+24+25+26+27+28+29+30\right)=330\)

\(\Rightarrow6x+\left[\left(30-20\right):1+1\right]\left(20+30\right):2=330\)

\(\Rightarrow6x+11.50:2=330\)

\(\Rightarrow6x+275=330\)

\(\Rightarrow6x=55\)

\(\Rightarrow x=\dfrac{55}{6}\)

x + 20 + 21 + x + 22 + 23 + x + 24 + 25 + x + 26 + 27 + x + 28 + 29 + x + 30 = 330

6x + (30 + 20) . (30 - 20 + 1) : 2 = 330

6x + 50 . 11 : 2 = 330

6x + 275 = 330

6x = 330 - 275

6x = 55

x = 55 : 6

x = 55/6

a) \(x=-8\)

b) \(x=30\)

c) \(x=-4\)

25x - x = 312

x ( 25 - 1 ) = 312

24x           = 312

    x           = 13

( x + 20 ) + ( x + 30 ) + ( x + 50 ) + x = 300

( x + x + x + x ) + ( 20 + 30 + 50 )     = 300

4x + 100                                          = 300

4x                                                    = 200

 x                                                     = 50

\(25\times20\times125\times8-8\times30\times5\times125\\ =125\times500\times8-8\times150\times125\\ =125\times4000-1200\times125\\ =125\times\left(4000-1200\right)\\ =125\times2800\\ =350000\)