7. (10 points) Fill in each of the 3 “ $\square$ ” with an operation symbol so that the calculation result is a prime number. There are $\qquad$ different ways to do this. (No parentheses are allowed)
2 $\square$ $\square$ 7
【Answer】Solution: Enumerate one by one:
(1) $2+3+5-7=3$.
(2) $2+3-5+7=7$.
(3) $2-3+5+7=11$.
(4) $2 \times 3 \times 5-7=23$.
(5) $2+3+5+7=17$.
(6) $2+3 \times 5 \times 7=107$.
(7) $2 \times 3 \times 5+7=37$.
(8) $2 \times 3+5 \times 7=41$.

Therefore, the answer is: 8