To find the ones digit of the first prime number, a, in an arithmetic sequence of four primes with common difference 6, we can consider the possible ones digits of prime numbers greater than 5, which are 1, 3, 7, and 9. 

Adding 6 to each of these digits and taking the resulting ones digit, we get the sequences 1, 7, 3, 9; 3, 9, 5, 1; 7, 3, 9, 5; and 9, 5, 1, 7. 

Only the first sequence doesn't contain 5, so it's possible that the four primes have ones digits that form this sequence. Therefore, the ones digit of the first prime, a, is 1. For example, a could be equal to 11 to form the sequence of four consecutive primes {11, 17, 23, 29}.
Quattro numeri primi a, b, c e d formano una sequenza aritmetica crescente con a &gt; 5 e differenza comune 6. Qual è la cifra delle unità di a?