Here are the solutions for the problems:

1. 8 is what % of 40?

We use the formula: $\text{Percentage} = \left(\frac{\text{Part}}{\text{Whole}}\right) \times 100$ $\text{Percentage} = \left(\frac{8}{40}\right) \times 100 = 20\%$

Answer: 8 is 20% of 40.

---

2. What is 42% of 50?

We use the formula: $\text{Part} = \text{Percentage} \times \text{Whole}$ $\text{Part} = 0.42 \times 50 = 21$

Answer: 42% of 50 is 21.

---

3. 12 is 10% of what number?

We use the formula: $\text{Whole} = \frac{\text{Part}}{\text{Percentage}}$ $\text{Whole} = \frac{12}{0.10} = 120$

Answer: 12 is 10% of 120.

---

4. Find 60% of 20.

We use the formula: $\text{Part} = \text{Percentage} \times \text{Whole}$ $\text{Part} = 0.60 \times 20 = 12$

Answer: 60% of 20 is 12.

---

Would you like additional details or practice on this topic? Here are 5 related questions for further practice:

1. What is 30% of 90?
2. If 25 is 50% of a number, what is the number?
3. Find 15% of 200.
4. 9 is what percentage of 45?
5. A number is 5% of 400. What is the number?

Tip: When calculating percentages, convert the percentage to a decimal by dividing by 100. This simplifies calculations.