The two points can be made into a right triangle with the two sides defined by two lines drawn from the two given points. Use the Pythagorean Theorem to solve for the hypotenuse, the distance between the two points. See attached graph.
2 = 25%. 25% also means 1/4. So, it is a quarter of something. So. So find 100%, you have to multiply 2 by 4. 2 * 4 = 8. Roman has 8 pencils in his backpack.