A boss handed her secretary a manuscript and asked for it to be typed up. 

“How quickly do you think you can do it?” The boss asked.

“Well, I’ll type an average of 20 pages per day” replied the secretary.

The secretary was rather lazy to start with and typed the first half at 10 pages per day. To make up for it the secretary typed the second half at 30 pages per day. 

“See, I did average 20 ages per day” declared the secretary. “Half of 10 + 30 is 20.”

“No you didn’t” said the boss. “I’m going to miss my deadline.”

Who was right?