How to make a rubber square in a div with a given height?

Question

css

How to make a rubber square in a div with a given height?
css
.header_menu {width:100%; height: 10%; background-color: white;}
.header_box {width:?; height: 100%; float:right; background-color: black;}

Answer 1

it? jsfiddle.net/1pnacsj7/1