According to the formula: S=VT

Therefore, it is concluded that Vprimitive =S/T=S/8.

Increase by one third: after V =S/8*( 1+ 1/3).

Time to go is: t = s/v = 8/(1+1/3) = 6 hours.

So the round trip time is 2*6= 12 hours.

Grade 2.5: 380*[220/(220+ 160)]=220 trees.

Grade 4: 380 * [160/(220+160)] =160 strains.