/* Mặt hồ */
.river-arena {
    height: 160px;
    background: linear-gradient(180deg, #4dd0e1 0%, #00acc1 50%, #00838f 100%);
    box-shadow: inset 0 0 15px rgba(0,0,0,0.3);
    position: relative;
}

.water-wave {
    position: absolute;
    top: 0; left: 0; width: 100%; height: 100%;
    background-image: repeating-linear-gradient(0deg, transparent, transparent 15px, rgba(255,255,255,0.1) 15px, rgba(255,255,255,0.1) 16px);
    z-index: 1;
}

.river-bank {
    position: absolute;
    right: 0; top: 0; bottom: 0; width: 40px;
    background-color: #8d6e63;
    border-left: 3px solid #5d4037;
    z-index: 2;
    writing-mode: vertical-rl;
    text-orientation: upright;
    color: white;
    font-weight: bold;
    text-align: center;
    padding-right: 5px;
}

/* Lá sen */
.lotus-leaf {
    position: absolute;
    font-size: 2rem;
    z-index: 3;
    filter: drop-shadow(0 2px 2px rgba(0,0,0,0.4));
    transform: translate(-50%, -50%);
}

/* Ếch xanh */
.frog {
    position: absolute;
    font-size: 2.5rem;
    z-index: 4;
    filter: drop-shadow(0 3px 3px rgba(0,0,0,0.5));
    transform: translate(-50%, -50%);
    /* Hiệu ứng nhảy mượt */
    transition: left 0.6s linear, top 0.6s cubic-bezier(0.25, 1, 0.5, 1);
}

/* Rung lắc khi sai */
.shake-error { animation: splash 0.4s; }
@keyframes splash {
    0% { transform: translate(-50%, -50%) rotate(0deg); }
    25% { transform: translate(-50%, -50%) rotate(-15deg) scale(0.9); }
    50% { transform: translate(-50%, -50%) rotate(15deg) scale(1.1); }
    75% { transform: translate(-50%, -50%) rotate(-15deg) scale(0.9); }
    100% { transform: translate(-50%, -50%) rotate(0deg); }
}

/* CSS Định dạng Phân số chuẩn (Ví dụ: 1/3 có gạch ngang) */
.frac {
    display: inline-flex;
    flex-direction: column;
    align-items: center;
    vertical-align: middle;
    line-height: 1;
    font-weight: 900;
    color: #c62828;
    margin: 0 4px;
}
.frac .f-top { padding: 0 3px; }
.frac .f-bot { padding: 0 3px; border-top: 2px solid #c62828; }

.min-h-75 { min-height: 85px; }
.hl-num { color: #d84315; font-weight: 900; font-size: 1.2em; }