First find the total number of pages read
12 +8 = 20 pages
We can use ratios to determine how long it will take
6 pages 20 pages
------------ = -----------
21 minutes x minutes
Using cross products
6 * x = 21 * 20
6x = 420
Divide each side by 6
6x/6 = 420/6
x =70
It will take 70 minutes.