Solution: Suppose the original plan was to dig Y cubic meters per hour and complete the task in X hours.

There is xy=220①.

3y+(y+ 10)(x-2-3)=220+20②

It is simplified from ② to -2y+xy+ 10x=290.

Substitute ① into -2y+ 10x=70, that is 5x-y=35③.

Substituting ③ into ① gives x(5x-35)=220, and x 1=-4 (omitted) x2= 1 1.

Substitute x= 1 1 into ① to get y=20.

So the original plan was to dig 20 cubic meters per hour.