【評論主題】已知3907.245×658.198=2571740.845 若 3907.245×668.198= X，3807.245×658.198=Y，則X-Y會等於多少呢？(A)90 (B)2314566.

【評論內容】將3807.245×658.198=Y化作R x r則X　=(R+100) x (r+10)   　　=Rr+10R+100r+1000X-Y=Rr+10R+100r+1000　-　Rr   　　=10R+100r+1000   　　=(10x3807.245)+(100x658.198)+1000   　　=答案