Solve 5x − 4 = 81

The solution of 5x − 4 = 81 is x = 17.

5x - 4 = 81\ \Rightarrow\ x = 17

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The lines y = 5x − 4 and y = 81 meet at x = 17. 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.
$5x - 4 = 81$
2
Add 4 to both sides:
$5x - 4 + 4 = 81 + 4 \ \Rightarrow\ 5x = 85$
3
Divide both sides by 5:
$\frac{5x}{5} = \frac{85}{5} \ \Rightarrow\ x = 17$
4
Check: put x = 17 back into the equation.
$5\cdot17 - 4 = 85 - 4 = 81 \checkmark$

LaTeX source of the formulas on this page

\[ 5x - 4 = 81\ \Rightarrow\ x = 17 \]
\[ 5x - 4 = 81 \]
\[ 5x - 4 + 4 = 81 + 4 \ \Rightarrow\ 5x = 85 \]
\[ \frac{5x}{5} = \frac{85}{5} \ \Rightarrow\ x = 17 \]
\[ 5\cdot17 - 4 = 85 - 4 = 81 \checkmark \]

Trying values of x in 5x − 4 = 81

x	5x − 4	Equals 81?
15	71	no
16	76	no
17	81	yes
18	86	no
19	91	no

Questions

What is x in 5x − 4 = 81?

The solution of 5x − 4 = 81 is x = 17.

How do you solve 5x − 4 = 81?

Add 4 to both sides to get 5x = 85, then divide both sides by 5 to get x = 17.

How do I check the answer?

Replace x by 17: 5(17) − 4 = 81, which is 81.

Solve 5x − 4 = 81

How to work it out

Trying values of x in 5x − 4 = 81

Questions

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