I believe your reading is correct. The problem is stating "Misha lacked one kopeck". Consider this solution:

  Masha has 0 kopecks
  Misha has 6 kopeks
  The book costs 7 kopeks

Granted, you could consider this a trick question in this case because it doesn't really make sense to "combine" money with someone who didn't have any. I am not sure if this is the case, but if a kopeck is sub-dividable, you could also have the solution:

  Masha has .5 kopecks
  Misha has 6.5 kopeks
  The book costs 7.5 kopeks