1 be nuoc co hai voi vao .voi 1 chay 4 1/2 gio thi day be . voi 2 chay 4 gio 45 phut thi day be .luc dau nguoi ta mo voi 1 chay mot thoi gian =thoi gian ca 2 voi cung chay de day be ,sau do nguoita cho 2 voi cung chay de day be .hoi tu khi voi thu 2 cung chay thi bao lau day be