Create a proportion.
Since it takes 7 minutes to print 63 pages, and half an hour is 30 minutes, you create two ratios(unit per unit) and line up the units(minutes to minutes, pages to pages).
Now you cross multiply, 63 * 30 = 1890 and 7 * x = 7x.
So we have 1890 = 7x.
Then we divide both sides by seven.
270 = x, therefore you can print 270 pages in half an hour :)