Arrange 1/2, 2/5, 5/8 and 7/10 in ascending order, from the smallest to biggest.

The denominators are 2, 5, 8, and 10. The least common multiple (LCM) of these numbers is 40.

The fractions are $\frac{16}{40}, \frac{20}{40}, \frac{25}{40}, \frac{28}{40}$.
Converting back to the original fractions: $\frac{2}{5}, \frac{1}{2}, \frac{5}{8}, \frac{7}{10}$.

Option B is $\frac{2}{5}, \frac{1}{2}, \frac{5}{8}, \frac{7}{10}$.
The correct option is B.

10. Simplify $\frac{2}{3} + \frac{3}{6} \div 2\frac{1}{2}$

The expression becomes: $\frac{2}{3} + \frac{1}{2} \div \frac{5}{2}$

Find a common denominator, which is 15.

The calculated answer is $\frac{13}{15}$. This answer is not among the given options (A. $\frac{4}{3}$, B. $1\frac{1}{3}$, C. 1, D. $\frac{3}{5}$, E. $\frac{1}{3}$). There might be an error in the question or the provided options. Based on standard mathematical operations, $\frac{13}{15}$ is the correct simplification. Since I must select an option, and none match, I cannot provide a definitive answer from the given choices.

11. Some bricklayers working on the fencing of a school compound took 10 days to complete $\frac{2}{5}$ of the job. If they are working at the same rate, how many more days will they spend to finish the job?

The bricklayers completed $\frac{2}{5}$ of the job in 10 days.
This means they complete $\frac{2/5}{10} = \frac{2}{50} = \frac{1}{25}$ of the job per day.

The total job is 1 (or $\frac{5}{5}$).
Remaining job = $1 - \frac{2}{5} = \frac{5}{5} - \frac{2}{5} = \frac{3}{5}$.

Number of days = $\frac{\text{Remaining job}}{\text{Rate of work per day}}$

The correct option is B.

12. Which of the following is equal to $\frac{3}{8}$?

A. $\frac{3+3}{8+3} = \frac{6}{11}$
B. $\frac{3-1}{8-1} = \frac{2}{7}$
C. $\frac{3 \times 2}{8 \times 3} = \frac{6}{24} = \frac{1}{4}$
D. $\frac{3 \div 4}{8 \div 4} = \frac{3/4}{2} = \frac{3}{4} \times \frac{1}{2} = \frac{3}{8}$
E. $\frac{3+1}{8+1} = \frac{4}{9}$

Option D is equal to $\frac{3}{8}$.
The correct option is D.

13. Simplify $\frac{0.3 \times 2.4}{0.6 \times 1.2}$

The correct option is E.

14. Find the sum of 18.206, 200 and 1.02.

The sum is 219.226.
The correct option is A.

15. Express 300cm, as a decimal of 3 km.

1 km = 1000 m
3 km = $3 \times 1000 \text{ m} = 3000 \text{ m}$
1 m = 100 cm
300 cm = $\frac{300}{100} \text{ m} = 3 \text{ m}$

The correct option is D.

16. Convert $\frac{1}{8}$ to a decimal number.

The correct option is D.

17. If $1.00 = \text{N}7.3217$, how much will a trader pay for $100.00?

Given: $1.00 = \text{N}7.3217$
To find the cost for $100.00, multiply the rate by 100.

The question asks "how much will a trader pay for $100.00?". The answer is N732.17. There are no options provided for this question.