If Tom could triple his reading speed, he could read 12 * 3 = 36 pages of a book in one hour. Therefore, during two hours he could read 36 * 2 = 72 pages.
This justifies what answer for what question? Q & A:
During one hour, Tom can read 12 pages of a book. How many pages would he be able to read during 2 hours if he could increase his reading speed by a factor of 3?
72