how to keep ratio of the element's width and height when window size changes?

Question

I have a IMG with style like this:

position: absolute;
left: 50px;
top: 30px;
width: 150px;
height: 120px;

So, this IMG will be displayed with ratio of width and height equal to 5/4. How can I make the ratio still be 5/4 when the window size changes?