G7.4 If the unit digit of $7^{14}$ is $d$, find the value of $d$.
$7^{1}=7,7^{2}=49,7^{3}=343,7^{4}=2401$; the unit digits repeat in the pattern $7,9,3,1, \ldots$
$$
7^{14}=\left(7^{4}\right)^{3} \times 7^{2} \therefore d=9
$$