a) \(\left(x+1\right)+\left(x+4\right)+\left(x+7\right)+...+\left(x+28\right)=155\)
\(\Rightarrow10\times x+\left(1+4+7+...+28\right)=155\)
\(\Rightarrow10\times x+\frac{\left(1+28\right)\times10}{2}=155\)
\(\Rightarrow10\times x+145=155\)
\(\Rightarrow10\times x=10\Rightarrow x=1\)
b)
b, 71 + 65 x 4 = x + 140 x + 260
71 + 260 =( x + 140) : x + 260
71 = (x + 140) : x
70 x x : x = x+ 140
70 x x = 140
x = 140 : 70
x=2