Solve 8x − 15 = 129

The solution of 8x − 15 = 129 is x = 18.

8x - 15 = 129\ \Rightarrow\ x = 18

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The lines y = 8x − 15 and y = 129 meet at x = 18. The solution of the equation is the x-coordinate of the point where the two sides are equal.

How to work it out

1
The goal is to get x alone. Undo the operations in the opposite order: first the subtraction, then the multiplication.
$8x - 15 = 129$
2
Add 15 to both sides:
$8x - 15 + 15 = 129 + 15 \ \Rightarrow\ 8x = 144$
3
Divide both sides by 8:
$\frac{8x}{8} = \frac{144}{8} \ \Rightarrow\ x = 18$
4
Check: put x = 18 back into the equation.
$8\cdot18 - 15 = 144 - 15 = 129 \checkmark$

LaTeX source of the formulas on this page

\[ 8x - 15 = 129\ \Rightarrow\ x = 18 \]
\[ 8x - 15 = 129 \]
\[ 8x - 15 + 15 = 129 + 15 \ \Rightarrow\ 8x = 144 \]
\[ \frac{8x}{8} = \frac{144}{8} \ \Rightarrow\ x = 18 \]
\[ 8\cdot18 - 15 = 144 - 15 = 129 \checkmark \]

Trying values of x in 8x − 15 = 129

x	8x − 15	Equals 129?
16	113	no
17	121	no
18	129	yes
19	137	no
20	145	no

Questions

What is x in 8x − 15 = 129?

The solution of 8x − 15 = 129 is x = 18.

How do you solve 8x − 15 = 129?

Add 15 to both sides to get 8x = 144, then divide both sides by 8 to get x = 18.

How do I check the answer?

Replace x by 18: 8(18) − 15 = 129, which is 129.

Solve 8x − 15 = 129

How to work it out

Trying values of x in 8x − 15 = 129

Questions

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