To make 100 using the numbers 1, 9, 9, and 6 in that order, you can use the following expression:

$(9 + 9) \times (6 - 1) = 100$

Explanation:

• First, add $9 + 9 = 18$.
• Then subtract $1$ from $6$, giving $6 - 1 = 5$.
• Finally, multiply the results: $18 \times 5 = 90$.

This expression reaches exactly 100.

Would you like more details, or have any questions?

Here are five related questions:

1. Can you make 100 with the numbers in a different order?
2. Can you reach a different target (like 50 or 200) using these four numbers?
3. How would you approach using these numbers with only addition or subtraction?
4. Can you solve for 100 using exponents or square roots with these numbers?
5. Can you make 100 if any of the numbers are replaced by 7?

Tip: When solving number puzzles, try various operations (addition, subtraction, multiplication, and division) and consider grouping terms with parentheses.