Chapter 5, Problem 1ECP

To determine

To solve: The equation $X-20=210$ for the unknown X.

Answer to Problem 1ECP

The value of the unknown variable X is 230.

Explanation of Solution

Result used:

Opposite process rule:

“If an equation indicates a process such as addition, subtraction, multiplication, or division, solve for the unknown or variable by using the opposite process.”

Calculation:

The given equation is $X-20=210$.

This equation means that, ‘Some number X less 20 equals 210.

Since the equation process used subtraction, use the opposite process rule and solve for the variable X with addition.

To isolate the variable X on the left, add 20 to both sides of the equation.

$\begin{array}{l}X-20+20=210+20\\ X+0=210+20\\ X=230\end{array}$

That is, $X=230$.

Check the result by substituting $X=230$ in the original equation.

$\begin{array}{l}X-20\stackrel{?}{=}210\\ \left(230\right)-20\stackrel{?}{=}210\\ 230-20\stackrel{?}{=}210\\ 210\stackrel{?}{=}210\left(\text{True}\right)\end{array}$

Thus, the value of the unknown variable X is 230.