To make 90 using exactly four 9s with the operations of addition, subtraction, multiplication, and division, you can use the following expression:

$99 - \frac{9}{9} = 90$

Here's how it works step-by-step:

1. Create 99 using two 9s: $99$

   • You can form 99 by combining two 9s: 9 followed by 9 gives you 99.

2. Divide 9 by 9 to get 1: $\frac{9}{9} = 1$

   • Using the remaining two 9s, you can divide one by the other, resulting in 1.

3. Subtract 1 from 99: $99 - 1 = 90$

   • Finally, subtract the result of the division (1) from 99 to get 90.

Thus, using the four 9s with the allowed operations, $99 - \frac{9}{9}$ gives you 90.

Would you like more details or have any questions about this solution? Here are some related questions to think about:

1. How can you express the number 100 using four 9s?
2. Can you find a way to make 91 using four 9s?
3. How many ways can you make the number 10 using four 9s?
4. Can you use four 9s to create a number less than 50?
5. What other numbers can you create using four 9s and basic arithmetic operations?

Tip: When solving these types of puzzles, remember that combining digits (like forming 99 from two 9s) and using parentheses can help you reach the desired result creatively.