[watertom]

The problem is that the girls can't afford to buy a book singularly.

Masha needs 7 kopecks to buy 1 book

"Misha lacked one", which means to me as an native English speaker that Misha needed just one more Kopeck in order to purchase 1 book.

When it's revealed that when they pool their money that they don't have enough to purchase just 1 book, I realized that the translation is faulty and stopped looking at the rest of the problems.

[sudobash1]

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