calculator {
    display: block;
}
explain {
    display: block;
    margin-top: 15px;
    padding-bottom: 15px;
}

calculator table {
    background-color: #cfcfcf;
    padding: 15px;
    margin: 0 auto;
    min-width: 300px;
    max-width: 300px;
}

calculator table tr td {
    padding-top: 15px;
}
calculator table tr:first-child td {
    padding-top: 0;
}
calculator table tr td:first-child {
    padding-right: 10px;
}
calculator table tr.button td:first-child {
    padding-right: 0;
}
calculator table tr td:last-child {
    text-align: right;
}
calculator table input[type="number"],
calculator table input[type="text"] {
    width: 75px;
}
calculator table input[type="button"] {
    width: 100%;
}

calculator table #error {
    visibility: hidden;
    color: red;
}
calculator table #error.active {
    visibility: visible;
    color: red;
}

@media screen and (min-width: 426px) {
    calculator {
        display: flex;
    }
    explain {
        margin-top: 0;
        margin-left: 15px;
    }
}