Find the sum of the first seven prime numbers that have a units digit of 7.
The smallest number that ends in 7 is 7 itself, and that is prime, so that's the first one. The next number that ends in 7 is 17, and that is also prime, because it is not divisible by any number between 2 and 16. The next number that ends in 7 is 27, but that is not prime, because it is divisible by 3 and 9. The next number that ends in 7 is 37, and that is prime, because it is not divisible by any number between 2 and 36. The next number that ends in 7 is 47, and that is also prime, because it is not divisible by any number between 2 and 46. The next number that ends in 7 is 57, but that is not prime, because it is divisible by 3 and 19. The next number that ends in 7 is 67, and that is prime, because it is not divisible by any number between 2 and 66. The next number that ends in 7 is 77, but that is not prime, because it is divisible by 7 and 11. The next number that ends in 7 is 87, but that is not prime, because it is divisible by 3 and 29. The next number that ends in 7 is 97, and that is prime, because it is not divisible by any number between 2 and 96. The next number that ends in 7 is 107, and that is also prime, because it is not divisible by any number between 2 and 106. So far, I have found seven prime numbers that end in 7: 7, 17, 37, 47, 67, 97, and 107. To find their sum, I can add them up using the column method: 7 + 17 + 37 + 47 + 67 + 97 + 107 = 379. 