How to retain responsive navigation menu in twentyeleven child theme

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Try to edit the line “display:inline-block !important;” to “display:none;” and keep the rest as it is.

.main-navigation ul.nav-menu, .main-navigation div.nav-menu > ul {
    border-bottom: 0 solid #EDEDED;
    border-top: 0 solid #EDEDED;
    display: none;
    margin-left: 28%;
    margin-top: 0;
    text-align: left;
    width: 100%;
}