The answer is C. Supplementary.
Si it is a linear eqaution
The mean is the average of the numbers: a calculated "central" value of a set of numbers.
Therefore: If the amount spent on books per month by all her classmates is leveled that amount would be $30
To calculate it: add up all the numbers, then divide by how many numbers there are
Answer: 15 boys
Explanation: ratio 5:7
5 + 7 = 12
Number of boys = 5/12 × 36