.layers-animation img:nth-child(4):hover {
    transform: translate(180px, -200px);
    transition: 0.5s;

    /* I want to be able to do this V */

    .layers-animation img {
        opacity: 0.5;
        transition: 0.5s;
    }

    /* (Change the opacity of all the other images when this image is hovered over.) */

}