1) We can write down all the prime numbers and check if they have 1 as a unit digit. Then we count them.
2) This is quicker way. We write down all the numbers that are less than 100 and that have 1 as a unit digit: 1 11 21 31 41 51 61 71 81 91 Now we check if each of them is prime number. By definition 1 is not prime number. From our list prime numbers are: 11,31,41,61,71.
So, we have total of 5 prime numbers that are less than 100 and that have 1 as a unit digit.
