Question 169
Find all eight-digit numbers formed only with the digits $1$, $2$, $3$ and $4$, each twice, that:
a. there is a single digit between the two digits $1$;
b. There are two digits between the two digits $2$;
c. There are three digits between the two digits $3$;
d. There are four digits between the two digits $4$.
a. there is a single digit between the two digits $1$;
b. There are two digits between the two digits $2$;
c. There are three digits between the two digits $3$;
d. There are four digits between the two digits $4$.