IDE avoiding math operation
In the C family of programming languages, the division operator (/),
when used with integer arguments, provides an integer division. You
get the integer quotient, and you can use % to get the remainder.

If you want a floating point division, then at least one of the
arguments should be a floating point number. E.g.:



Serial.print(valor / 700.0 * 255);


(the DEC argument is redundant). The final result is then truncated to
an integer before being passed to Serial.print().

A better option, which is outlined in dhimaspw's answer, is to do the
multiplication before the division. This way you avoid costly floating
point operations. If you do that, however, you should be careful about
overflows: valor*255 can be as big as 1023&nbsp;×&nbsp;255
=&nbsp;260865, which is larger than the maximum value an int can store
(215−1 =&nbsp;32767). You should then write

Serial.print(valor * 255L / 700);


where the L suffix forces the operation to be carried using long
integers.

The simplest solution, however, is to use the Arduino function map(),
which does essentially the same as above:

Serial.print(map(valor, 0, 700, 0, 255));