Those are ratios based on the y dim being set to 1 for easy math, so - the x dim:y dim(set to 1). So just divide the x pixels by the first number to get the Y pixels.
Remember though that some formats. (like 2.35) often include a letter box. So while the image area might be 1920 x 820, the frame size may still be 1920 x 1080 with the area beyond. The image area just being black horizontal strips.