I converted my wordpress theme to responsive.. all values of "width" change to percents (%)

The problem is:

from 400px width it show:

first line 2 post

second line just one post (instead 2 post)..

how can I fix that so the page will show 2 post in all line?

I attached some pictures to explain the situation.

image of smartphone until 400px width

image of 500px