What is 63.5 if you estimate it?

Answer 1

The front end estimate is 60. But estimating a single number, in isolation and without context, is a complete waste of time. It all comes down to the context of what needs to be done with the number.

If I had to add 63.5 to 1 trillion, I would estimate it as 0.
If I had to add 63.5 to 0.0000000001, I would use the number as given, or possibly the front end estimate. If I had to divide it by 4, I would estimate it as 64, because 64/4 = 16. If I had to divide it by 5, I would estimate it as 65, because 65/5 = 13.

Answer 2

To the nearest ten, 60