Can I make my slider proportionally?

Hi there, I have a question about “SLIDER”.
My slider is full width. You can check my site. My site is here.
I would like to make my slider’s height to width’s 50% (1/2) in all devices.

ex) If width:1000px = height:500px
If width:600px = height:300px

This is code for slider height:

.posts-slider .slides li {
	height:685px;
}

How can I modify it to make slider’s height into width’s 50% in all devices?

Thank you so much:)