.f-0-0-300 {
    flex: 0 0 300px;
}
ins.adsbygoogle {
    display: block;
    margin: 0 auto;
    width: 300px;
    height: 250px;
}

@media (min-width: 768px) {
    .col-md-300 {
        flex: 0 0 300px;
        max-width: 300px;
    }
}