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Is possible to save a Mathcad 15 worksheet with all the plots so as to avoid the need for recalculating all the plots when the file is reopened
Not as a Mathcad worksheet, no. You can copy and paste the plots into the worksheet as a bitmap, or you can print the entire worksheet to a pdf.
You can also save your worksheet with plots as a web page (in main menu select "File" -> "Save as Web Page").