Any odd positive integer $a$ is of the form . . . . . . ,where $m$ is an integer.

For any positive integer $n$,the last digit of $5^n$ is always.......

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.

The last digit of $5^{25}$ is . . . . . . .

Prove that $21^{n}$ cannot end in zero for any natural number $n \in N$.

State whether the following rational number has a terminating decimal expansion or not. If it has a terminating decimal expansion,find it: $\frac{77}{210}$