To find out how many pages Bertha read altogether, you can multiply the number of books by the number of pages in each book.
Bertha read 21 books, and each book had 155 pages:
\[ 21 \text{ books} \times 155 \text{ pages/book} = 3,255 \text{ pages} \]
So, Bertha read a total of 3,255 pages.