A two-digit number is such that the product of its digits is 20. When the digits are reversed, the number formed is greater than the original number by 9. Find the two-digit number.

Let the two-digit number be $10t + u$, where $t$ is the tens digit and $u$ is the units digit.

Condition 1: The product of its digits is 20.
$$
t \times u = 20 \quad (1)
$$
Condition 2: When the digits are reversed, the number formed is greater than the original number by 9.  
The reversed number is $10u + t$.
$$
10u + t = (10t + u) + 9
$$
$$
10u + t = 10t + u + 9
$$
$$
9u - 9t = 9
$$
Divide by 9:
$$
u - t = 1 \quad (2)
$$

From equation (2), express $u$ in terms of $t$:
$$
u = t + 1
$$
Substitute this into equation (1):
$$
t(t + 1) = 20
$$
$$
t^2 + t = 20
$$
$$
t^2 + t - 20 = 0
$$

Factor the quadratic equation:
$$
(t + 5)(t - 4) = 0
$$
This gives two possible values for $t$: $t = -5$ or $t = 4$.  
Since $t$ must be a positive digit, we take $t = 4$.

Using $u = t + 1$:
$$
u = 4 + 1 = 5
$$

The number is $10t + u = 10(4) + 5 = 45$.  
The two-digit number is $\boxed{\text{45}}$.

Daily rate = Sh 2500  
Number of days = 5
$$
\text{Cost for days} = 2500 \text{ Sh/day} \times 5 \text{ days} = 12500 \text{ Sh}
$$

Total kilometers driven = 40 km  
Free kilometers per day = 30 km  
Total free kilometers for 5 days = $30 \text{ km/day} \times 5 \text{ days} = 150 \text{ km}$  
Since the total kilometers driven (40 km) is less than the total free kilometers (150 km), there is no charge for the kilometers.
$$
\text{Cost for kilometers} = 0 \text{ Sh}
$$

$$
\text{Total Cost} = \text{Cost for days} + \text{Cost for kilometers}
$$
$$
\text{Total Cost} = 12500 \text{ Sh} + 0 \text{ Sh} = 12500 \text{ Sh}
$$
The total cost is $\boxed{\text{Sh 12500}}$.

The standard equation of a circle with center $(h, k)$ and radius $r$ is $(x - h)^2 + (y - k)^2 = r^2$.  
Given center $P(5, 5)$, so $h=5$, $k=5$.  
Given radius $r=5$.

$$
(x - 5)^2 + (y - 5)^2 = 5^2
$$

$$
(x^2 - 10x + 25) + (y^2 - 10y + 25) = 25
$$

$$
x^2 + y^2 - 10x - 10y + 25 + 25 - 25 = 0
$$
$$
x^2 + y^2 - 10x - 10y + 25 = 0
$$
The equation of the circle is $\boxed{x^2 + y^2 - 10x - 10y + 25 = 0}$.

b) The point A is not provided in the question or any accompanying diagram. Therefore, the gradient of PA cannot be determined.

c) The point Q is not provided in the question or any accompanying diagram. Therefore, the equation of the tangent at Q cannot be determined.