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.
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
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.