To find midpoint between points $ A(x_1,y_1) $ and $ B(x_2,y_2) $, we use formula:

$$ M = \left( \frac{x_1 + x_2}{2}, \frac{y_1 + y_2}{2} \right) $$

In this example we have:

$$ \begin{aligned} & A \left(20,~11\right) \implies x_1 = 20 ~~\text{and}~~ y_1 = 11 \\[1 em] & B \left(0,~-10\right) \implies x_2 = 0 ~~\text{and}~~ y_2 = -10 \end{aligned} $$

After substituting into above formula, we get:

$$ \begin{aligned} M & \left( \frac{ 20 + 0 }{2}, \frac{ 11 + \left( -10\right) }{2} \right) \\[1 em] M & \left( \frac{ 20 }{2}, \frac{ 1 }{2} \right) \\[1 em] M & \left(10,~\dfrac{ 1 }{ 2 }\right) \end{aligned} $$

Midpoint Calculator