Solve the following number sentences for the unknown:

$$1172 - \square = 271$$ $$\square = 1172 - 271$$

$$\square = 901$$

The correct value is $\boxed{\text{901}}$.

2.

$$\square \times 3 = 396$$ $$\square = \frac{396}{3}$$

$$\square = 132$$

The correct value is $\boxed{\text{132}}$.

3.

$$42 \div \square + 3 = 9$$ $$42 \div \square = 9 - 3$$ $$42 \div \square = 6$$

$$\square = 7$$

The correct value is $\boxed{\text{7}}$.

4.

$$8 \times \square = 4$$ $$\square = \frac{4}{8}$$

$$\square = \frac{1}{2}$$

The correct value is $\boxed{\frac{1}{2}}$ or $\boxed{0.5}$.

5.

(Assuming 13,8 means 13.8)

$$13.8 + \square = 28$$ $$\square = 28 - 13.8$$

$$\square = 14.2$$

The correct value is $\boxed{\text{14.2}}$.

6.

$$15 = 2 \times \square + 1$$ $$15 - 1 = 2 \times \square$$ $$14 = 2 \times \square$$

$$\square = 7$$

The correct value is $\boxed{\text{7}}$.

7.

$$\frac{2}{3} \times \square = 1$$ $$\square = 1 \div \frac{2}{3}$$

$$\square = 1 \times \frac{3}{2}$$ $$\square = \frac{3}{2}$$

The correct value is $\boxed{\frac{3}{2}}$ or $\boxed{1.5}$.

8.

$$1.5 + \square = 4$$ $$\square = 4 - 1.5$$

$$\square = 2.5$$

The correct value is $\boxed{\text{2.5}}$ or $\boxed{2\frac{1}{2}}$.

9.

$$\square^2 = 49$$ $$\square = \sqrt{49}$$

$$\square = 7$$

The correct value is $\boxed{\text{7}}$.