The countercurrent time is (35+5)÷2=20 hours.

Downstream time is 35-20= 15 (hours)

So the countercurrent velocity is 360÷20= 18 (km/h).

Downward speed is 360÷ 15=24 (km/h).

So the water speed is (24- 18)÷2=3 (km/h) (downstream speed = boat speed+water speed, upstream speed = boat speed-water speed).

The speed of this sailboat in the water is 12+3= 15 (km/h).

The velocity in the current is 12-3=9 (km/h).

So it needs 360÷ 15+360÷9=64 (hours).

The comprehensive formula is (35+5)÷2=20 (km/h).

[360÷20-360÷(35-20)]÷2=3 (km/h)

360 ÷ (12+3)+360 ÷ (12-3) = 64 (hours)

I hope my answer is helpful to you.