Estimates are typically whole numbers.
To estimate an answer, you would take whatever numbers you are trying to add/subtract/multiply/divide ect. and round them to the nearest whole number then complete said action with them.
Example: 3.99 + 33.85 + 31.74
3.99 is closest to 4
33.85 is closest to 34
and 31.74 is closest to 32
Then, we would add up all the numbers: 4 + 34 + 32 = 70
70 is very close to 69.58, therefore 70 is a good estimate.