If the child reads 12 pages in 18 minutes, that is 12/18 pages/minute, 12/18 can be reduced to 2/3 pages/minute by dividing top & bottom by 6. The rate is 2/3 page / minute (let that sink in: for every ONE minute, the child reads 2/3rd's of a page). Now given 30 minutes, the child can read 2/3 pages/minute * 30 minutes = 20 pages.
