a. (X -25 ) :15=20 b. (3 x X-15)x7=42 c.Xx(X+1)=1+4+6+8+10+...+2500 e.(X-1954)x5=50 f.30x(60-X)=30

Đáp án:

$\begin{array}{l}
a)\left( {x - 25} \right):15 = 20\\
 \Leftrightarrow x - 25 = 20.15\\
 \Leftrightarrow x - 25 = 300\\
 \Leftrightarrow x = 300 + 25\\
 \Leftrightarrow x = 325\\
Vậy\,x = 325\\
b)\left( {3.x - 15} \right).7 = 42\\
 \Leftrightarrow 3x - 15 = 42:7\\
 \Leftrightarrow 3x - 15 = 6\\
 \Leftrightarrow 3x = 6 + 15\\
 \Leftrightarrow 3x = 21\\
 \Leftrightarrow x = 21:3\\
 \Leftrightarrow x = 7\\
Vậy\,x = 7\\
c)x.\left( {x + 1} \right) = 2 + 4 + 6 + 8 + .... + 2500\\
 \Leftrightarrow x.\left( {x + 1} \right) = \dfrac{{\left( {2500 + 2} \right).1250}}{2}\\
 \Leftrightarrow x.\left( {x + 1} \right) = \dfrac{{2502}}{2}.1250\\
 \Leftrightarrow x.\left( {x + 1} \right) = 1251.1250\\
 \Leftrightarrow x = 1250\\
Vậy\,x = 1250\\
e)\left( {x - 1954} \right).5 = 50\\
 \Leftrightarrow \left( {x - 1954} \right) = 50:5\\
 \Leftrightarrow \left( {x - 1954} \right) = 10\\
 \Leftrightarrow x = 10 + 1954\\
 \Leftrightarrow x = 1964\\
Vậy\,x = 1964\\
f)30.\left( {60 - x} \right) = 30\\
 \Leftrightarrow 60 - x = 30:30\\
 \Leftrightarrow 60 - x = 1\\
 \Leftrightarrow x = 60 - 1\\
 \Leftrightarrow x = 59\\
Vậy\,x = 59
\end{array}$