The problem consists of the following questions:

51. 13 is what percent of 16?
53. 33 is what percent of 40?

To solve these, we use the formula for percentage:

$\text{Percentage} = \left(\frac{\text{Part}}{\text{Whole}}\right) \times 100$

For question 51:
We need to find what percent 13 is of 16.

$\text{Percentage} = \left(\frac{13}{16}\right) \times 100$

$\text{Percentage} = 0.8125 \times 100 = 81.25\%$

For question 53:
We need to find what percent 33 is of 40.

$\text{Percentage} = \left(\frac{33}{40}\right) \times 100$

$\text{Percentage} = 0.825 \times 100 = 82.5\%$

So, the answers are:
51. 13 is 81.25% of 16.
53. 33 is 82.5% of 40.

Would you like further details or explanations? Here are some additional questions you could explore:

1. How do you calculate the percentage of a number without using the formula?
2. What is the relationship between a percentage, a fraction, and a decimal?
3. How would the answers change if the whole values were larger or smaller?
4. What is the significance of percentages in real-world applications like finance?
5. How can you use percentages in calculating sales tax or discounts?

Tip: Always double-check your steps by converting the percentage back to the original fraction or decimal to ensure accuracy.