Please choose the correct ending to complete the given sentence: Education and Communications: [header] How to mentally calculate primality of any three digit number [title] Get pencil and paper. [step] Like any mathematical principle, you will remember it better if you practice on paper at the same time. [title] Estimate the square root of the number.

Ending1: [step] This will make your check faster since one only needs to try to divide the possible prime by the prime numbers less than the square root of that number. A quick reference might help: the square root of 100 is 10, of 225 is 15, of 400 is 20, of 625 is 25, and of 900 is 30. Ending2: [step] The square root of the number refers to how large it is; this is written first here. For example , 5 is 12 (24 = 24/36/70/55); 3 is 12 (36); and 4 is 12 (57). Ending3: [step] In case your new calculator isn't big enough to help you picture these numbers, make it a first step to first know its root. [substeps] Think " what/how is the square root of any number? " and google if you're in a circle. Ending4: [step] You don't need a calculator to get started. In fact, the math is easy : 5 = 400/250 x 900 = 1000.

Answer format: ending1/ending2/ending3/ending4