/kloc-quality table within 0/00
(1) formula:
Two, three, five, seven, eleven, thirteen are followed by seventeen, and nineteen. Don't forget, two, three, nine, four, four, three, four, seven, seven, seven, seven, seven, seven, nine, eight, three, eight, eight, nine, seventeen.
Prime number within 100
(2) with 2, 3, 5, 7 to try to divide, there is a remainder is a prime number:
Example: Are the numbers 77, 87 and 97 prime?
① Let's see: 77 ÷ 7 =11,which is a multiple of 7, so it is not a prime number; ②? Look at 87,87 ÷ 3 = 29, which is a multiple of 3 and not a prime number. We can judge whether 87 is a multiple of 3, or we can divide the sum of 8+7 by 3, which is ok. ③? Look at 97 again, its unit is 7, so it is not a multiple of 2 and 5. We just need to judge 3 and 7, and we will know that 97 is not a multiple of 3 and 7, so it is a prime number.