Answer:
773
Explanation:
prime numbers from 1 - 10
2,3,5,7.
The first digit should be 7.
7+5+3 = 15
7+3+3= 13 prime
7+5+2= 14
7+2+2= 11 prime
7+2+3= 12
7+5+5= 17 prime
7+7+7= 21
7+7+2= 16
7+7+3=17 prime and itself is prime
7+7+5=19 prime but 775 itself is not prime