Hello!
First, since you read 50 pages in 1 hour, 50/2 = 25 pages in 1/2 hour.
Then, your sister read 40 pages in 1 hour, 40/2 = 20 pages in 1/2 hour.
Then we subtract: 25-20 = 5 pages.
So: You read 5 pages more than your sister in 1/2 hour.
Hope this helped! c: