( 1)

Suppose the speed of the ship in still water is v,

2(v+3)=2.5(v-3)

V=27 km/h

That is, the speed of the ship in still water is 27 km/h.

(2)

Suppose x people are arranged to work first,

2x+8(x+5)=80×3/4

2x+8x+40=60

10x=20

X=2 people

Is to arrange two people to work first.