What is 0.90's 3 names like the 0.550 answer.

what a vague question. But three names for 0.90 might be

0.90
90/100
9/10
9.0 * 10^-1

To understand the names of 0.90, let's break down the number into its place values:

0.90 can be written as:
- 9 tenths (0.9)
- 90 hundredths (0.90)
- 900 thousandths (0.900)