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).
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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 :)