We assume the question asks to solve for x in the equation 2x + 4 = 8.

$$2x + 4 - 4 = 8 - 4$$ $$2x = 4$$

$$\frac{2x}{2} = \frac{4}{2}$$ $$x = 2$$

The value of $x$ is $\boxed{\text{2}}$.

2)
We need to solve for $x$ in the equation $300 \times 9x = 90,000$.

$$300 \times 9x = 2700x$$

So the equation becomes:

$$2700x = 90,000$$

$$x = \frac{90,000}{2700}$$ $$x = \frac{900}{27}$$

$$x = \frac{100}{3}$$

The value of $x$ is $\boxed{\frac{100}{3}}$.

3)
We are given the principal amount, the final amount, and the time, and we need to find the rate of simple interest.

Given:
Principal ($P$) = Rs. 12500
Amount ($A$) = Rs. 15500
Time ($T$) = 4 years

The simple interest is the difference between the amount and the principal.

$$SI = A - P$$ $$SI = \text{Rs. } 15500 - \text{Rs. } 12500$$ $$SI = \text{Rs. } 3000$$

The formula for simple interest is $SI = \frac{P \times R \times T}{100}$.
We can rearrange this formula to solve for $R$:

$$R = \frac{SI \times 100}{P \times T}$$

$$R = \frac{3000 \times 100}{12500 \times 4}$$ $$R = \frac{300,000}{50,000}$$ $$R = 6$$

The rate of interest is 6%.
Comparing this with the given options:
A. 3%
B. 4%
C. 5%
D. 6%

The correct option is D.
The rate of interest is $\boxed{\text{D. 6\%}}$.