Question

show that, except for 2 and 5, every prime number can be expressed as 10k+1, 10k+3 10k+7, 10k+9, where k is an interger

Answer 1

10(6)+1=61
10(15)+3=153
10(4)+7=47
10(9)+9=99

this is true because when you multiply a number by 10 it will be an even number(2,4,6,8,etc...)...then when you add an odd number to that the number will turn odd(153,47,99,etc..)... and when you multiple integers they will always multiply to an even number