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.