### Instruction: Find the sum of the first seven prime numbers that have a units digit of 7.### Response: The first thing I notice is that the problem is asking for the sum of prime numbers that end in 7, so I need to find those prime numbers first.
I recall that a prime number is a positive integer that has exactly two factors: 1 and itself.
I also recall that any number that ends in 0, 2, 4, 5, 6, or 8 is not prime, because it is divisible by 2, 5, or 10.
That means I only need to check the numbers that end in 1, 3, 7, or 9 for primality.
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.