Recognize this as an inverse proportion problem. More boys will take less time to load the same amount of cement.

More boys will take less time to load the same amount of cement.
Let $B$ be the number of boys and $T$ be the time in hours.
The product $B \times T$ is constant.

$B_1 = 2$ boys, $T_1 = 6$ hours
$B_2 = 5$ boys, $T_2 = ?$

It will take 5 boys $\boxed{\text{2.4 hours}}$ to load the same lorry of cement.

21. A tank of water of capacity 5000 litres serves a family for 16 days. How long will it take a water tank of 8000 litres to serve the same family?

A larger capacity tank will serve the family for more days.
Let $C$ be the capacity in litres and $D$ be the number of days.
The ratio $\frac{C}{D}$ is constant.

$C_1 = 5000$ litres, $D_1 = 16$ days
$C_2 = 8000$ litres, $D_2 = ?$

A water tank of 8000 litres will serve the same family for $\boxed{\text{25.6 days}}$.

22. 9 men working at the same rate and working 8 hrs in a day take 12 days to complete a given task. How long will it take six such men working 10 hrs in a day to complete the same task?

The total work done is constant.
Let $M$ be the number of men, $H$ be the hours worked per day, and $D$ be the number of days.
The total work units can be represented as $M \times H \times D$.

$M_1 = 9$ men, $H_1 = 8$ hours/day, $D_1 = 12$ days
$M_2 = 6$ men, $H_2 = 10$ hours/day, $D_2 = ?$

It will take six men working 10 hours a day $\boxed{\text{14.4 days}}$ to complete the same task.

23. What is the order of the matrix below?

There are 3 rows.

There are 2 columns.

The order of the matrix is $\boxed{3 \times 2}$.

24. Are the matrices below compatible?

It has 2 rows and 2 columns, so its order is $2 \times 2$.

It has 3 rows and 3 columns, so its order is $3 \times 3$.

For matrix addition or subtraction, the matrices must have the exact same order. Since the orders ($2 \times 2$ and $3 \times 3$) are different, the matrices are not compatible for subtraction.

The matrices are $\boxed{\text{not compatible}}$.

25. Work out

The result of the addition is $\boxed{\begin{pmatrix} 6 & 15 \\ 4 & -1 \\ 2 & 2 \end{pmatrix}}$.

26. Given $A=B-C$ and

Given $A = B - C$, we can write $C = B - A$.

The matrix $C$ is $\boxed{\begin{pmatrix} 1 & 6 & 6 \\ -5 & 1 & -3 \\ -2 & -1 & -3 \end{pmatrix}}$.