To solve this problem, we need to determine how long it will take Sue to do 100 math questions, given that she can do 20 math questions in 5 minutes.

Step 1: Find the rate at which Sue works.

Sue does 20 questions in 5 minutes. Therefore, the rate of working is: $\text{Rate} = \frac{20 \text{ questions}}{5 \text{ minutes}} = 4 \text{ questions per minute}$

Step 2: Find how long it takes to do 100 questions.

To find the time required for 100 questions, divide the total number of questions by the rate: $\text{Time} = \frac{100 \text{ questions}}{4 \text{ questions per minute}} = 25 \text{ minutes}$

So, it will take Sue 25 minutes to do 100 questions.

Answer: 25 minutes.

If you would like more details or have further questions, feel free to ask! Here are a few related questions:

1. How would the time change if Sue's rate of solving questions increased?
2. What would happen if Sue could only do 10 questions in 5 minutes instead of 20?
3. How could you calculate the time if the number of questions was doubled?
4. What is the time it takes Sue to complete 50 questions?
5. Can we use a similar method to solve for any other scenario with constant rates?

Tip: Always check if the rate is constant before applying formulas that use time, as this method assumes a steady rate of work.