Find?

divide the number by 2 and subtract 2from it

$\begin{aligned} 10-3&=7\\ 20-8&=12\\ 30-13&=17\\ 40-18&=22\end{aligned}$ The sequence $7,12,17,22$ is an arithmetic progression with initial term $a_{1}=7$ and common difference $d=5$ .Therefore its next term is $22+5=27$ .Hence: $50-?=27\implies ?=\boxed{23}$

For every 10 increase in the left hand, there is a 5 increase on the right hand of the equation. It's simple!