Can one use long float and unsigned float ? And why should we use double ?

Answer 1

Double stands for double-precision, which means that a double has more decimal places than a float (double 15, float 7). As far as I know, long-floats don't exist, nor do unsigned-floats (since floats follow the IEEE 754 standard). To briefly elaborate on unsigned data types, an integer's range is 4 bytes, and an unsigned integer's range is 4 bytes as well. Unsigned data types don't necessarily represent "larger" amounts, just "different" amounts.

Here's a link if you want to know more about floating point data-types:
https://en.wikipedia.org/wiki/IEEE_floating_point

Answer 2

yes and we use double to right numbers like 9.876