The title means that the current speed from Chongqing to Wuhan should be constant (Chongqing is in the upper reaches of Wuhan, which is geographical knowledge), and the speed of the ship is also constant.

Let the current speed be xkm/ day, the ship speed be ykm/ day, and the distance from Chongqing to Wuhan be skm.

5*(x+y)=s，( 1)

7*(y-x)=s，(2)

Obviously 5*(x+y)=7*(y-x),

So 5x+5y=7y-7x,

So 12x=2y,

So y=6x,

Substitute y=6x into (1) to get s=35x.

So it takes s/x=35x/x=35 days for a raft to drift from Chongqing to Wuhan.