Use the midpoint formula:

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

With endpoints $(30, 50)$ and $(90, 100)$:

$M = \left(\frac{30 + 90}{2},\; \frac{50 + 100}{2}\right) = \left(\frac{120}{2},\; \frac{150}{2}\right) = (60,\; 75)$