The last digit of $5^n$ is . . . . . . ,where $n \in N$.

Without performing long division,state whether the decimal expansion of the rational number $\frac{13}{3125}$ is terminating or non-terminating repeating.

Express $156$ as a product of its prime factors.

When $(6k+1)^2$ is divided by $6$,the remainder is . . . . . . .

Explain why $3 \times 5 \times 7 + 7$ is a composite number.

Explain why $7 \times 11 \times 13+13$ and $7 \times 6 \times 5 \times 4 \times 3 \times 2 \times 1+5$ are composite numbers.