Solution: (1) ∑ (x-x flat) (y-y flat) = ∑ xy-nx flat y flat =186.4-4x2.5x15.6 = 30.4.

∑ (X-X flat) 2 = ∑ X 2-NX flat 2 = 30-25 = 5

a=(nσxy-σxσy)/(nσx^2-(σx)^2)=(745.6-624)/( 120- 100)=6.08

B=y (average) -a*x (average) = 15.6-6.08x2.5=0.4.

y=6.08x+0.4

(2) Let x=5 and get y=30.8.

So it is reduced by 33-30.8=2.2 liters.

I can't remember the formula clearly. I don't know if it's right. Anyway, the least square method is to set the formula directly. It depends on whether you remember the formula or not.