71. If $[x]$ represents the greatest integer not greater than $x$, the equation $\left[\frac{x+a}{3}\right]=2$ has positive integer solutions for $x$, then the range of the constant $a$ is . $\qquad$
Answer: $a<8$

Solution: It is easy to know
$$
2 \leq \frac{x+a}{3}<3,
$$

Therefore,
$$
6-x \leq a<9-x \text {. }
$$

Since
$$
x \geq 1 \text {, }
$$

Thus,
$$
a<9-1=8 \text {. }
$$