Doi 1h20 phut = 1 va 1/3h = 4/3h
Ba voi nuoc chay vao be 1 gio thi duoc : 1:4/3 = 3/4 ( be )
Voi thu nhat chay vao be 1 gio thi duoc : 1:6 = 1/6 ( be )
Voi thu hai chay vao be 1 gio thi duoc : 1:4 = 1/4 ( be )
Vay voi thu ba chay vao be 1 gio thi duoc : 3/4- ( 1/6 + 1/4 ) = 1/3 ( be )
Vay voi thu ba sau : 1:1/3 = 3 ( gio )
Dap so : 3 gio