To find the prime numbers less than 100, I can start by listing the first few prime numbers and look for a pattern. The first prime numbers are 2, 3, 5, 7, 11, 13, 17, 19, 23, 29, 31, 37, 41, 43, 47, 53, 59, 61, 67, 71, 73, 79, 83, 89, and 97. That leaves me with the numbers ending in 1, 3, 7, or 9. I want to count how many of them have a units digit of 3. I see that there are 3, 13, 23, 43, 53, 73, and 83. That's 7 prime numbers less than 100 with a units digit of 3. 
How many prime numbers less than 100 have a units digit of 3?