One way to write 99 using four nines is (9x9) + (9+9). Now write 100 using four nines.

99 + 9/9

To write 100 using four nines, we can use the following expression:

(9 / 9) + (9 x 9)

Let's break it down:

1. The expression (9 / 9) evaluates to 1. When we divide 9 by 9, the result is 1.

2. The expression (9 x 9) evaluates to 81. When we multiply 9 by 9, the result is 81.

3. Adding 1 and 81 together gives us 82.

Therefore, by using the expression (9 / 9) + (9 x 9), we can write 100 using four nines.