<prompt> Translate the following text to EnglishKwa mfano, ikiwa idadi ya barua za matangazo ya kwanza hazijabadilika kati ya mwaka wa 1997 na 1998, tunapata takwimu zifuatazo kwa mwaka wa kifedha wa 1998:  <response>: 9 billion pieces respectively, and assuming that First-Class advertising mail volume has not changed between 1997 and 1998, we obtain the following figures for FY 1998: a 91. 