@media only screen and (max-width: 1280px) and (min-width: 981px) {
   
}
@media only screen and (max-width: 980px) and (min-width: 641px) {

    .pr li {
        width: 45%;
    }
}
@media only screen and ( max-width: 640px ) /* 640 */
{
    .pr li {
        width: 90%;
    }
}