Manuscript Pages

Mother asked vera to type up a manuscript. Vera decided “ I will type an average of 20 pages a day”. She typed the first part of the manuscript rather lazily, at 10 pages per day, but to make up for this she typed the second half at 30 pages a day. “See, I did 20 pages a day” said Vera to her mother:

(10+30)/2=20

“No, you didn’t” said her mother. Who was right?