Seven of the interior angles of an octagon are each (x+10)⁰ and the eight angle is 170⁰ Find the value of x...

The formula for the sum of interior angles of an $n$-sided polygon is $(n-2) \times 180^\circ$.
For an octagon, $n=8$.

$$\text{Sum of interior angles} = (8-2) \times 180^\circ = 6 \times 180^\circ = 1080^\circ$$

Seven angles are each $(x+10)^\circ$, and the eighth angle is $170^\circ$.

$$7(x+10) + 170 = 1080$$

$$7x + 70 + 170 = 1080$$ $$7x + 240 = 1080$$

Subtract 240 from both sides:

$$7x = 1080 - 240$$ $$7x = 840$$

Divide by 7:

$$x = \frac{840}{7}$$ $$x = 120$$

The value of $x$ is:

$$120$$