how do you add Time. example : 18.00 + 5 hours = 23.00 ??

If you mean how do you do it in Mathcad, see here: Date Calendar and Time functions.mcd - PTC Community

If you don't mean using Mathcad then you need to give more information

For time in Prime or Mathcad (<=15), you can use the built-in units system for basic conversions:

eg, just type '18hr+5hr=' which will give you the total time in seconds (the default time unit).  To see it in hours, click on the result; you should now see a small black square at the end of the expression (the place holder).  Type 'hr', click outside the expression and Mathcad should show the total time in hours.  To see the result in minutes, type 'min' instead of 'hr'.   Note when displaying time in hours that fractions are decimal fractions of an hour not minutes.

Stuart