Just for the sake of understanding, let us number computers from 1 to \(10\) and pendrives from 1 to 3. Initially only \(1st\) computer has the pdf.

\(1st\) minute: Roy copies the pdf from \(1st\) computer to 1st pendrive, so 1 computer and 1 pendrive has the pdf 2nd minute: He copies the pdf from \(1st\) pendrive to \(2nd\) computer and alongside he copies pdf from \(1st\) computer to \(2nd\) pendrive. \(3rd\) minute: Now he has two pendrives with pdf, so he copies the pdf to \(3rd\) and \(4th\) computer using \(1st\) and \(2nd\) pendrive. Alongside he already copied pdf to \(1st\) and \(2nd\) computer and has one pendrive blank. So he copies pdf into 3rd pendrive using any of the \(1st\) or \(2nd\) computer. \(4th\) minute: Now first 4 computers are done and we have pdf in all 3 pendrives. So he simply copies pdf into \(5th\), \(6th\) and \(7th\) computer using three pendrive respectively. \(5th\) minute: He copies the pdf into \(8th\), \(9th\) and \(10th\) computers using three pendrives respectively.