I wonder if there's an issue in translation. How could they "combine" their money if Masha had nothing to begin with? But if we assume quantities of money can be non-integers, then the problem is underconstrained.
I presume they used kopecks for a purpose and not rubles, dollars or another unit that can be subdivided. That forces the integerness of the amounts and thus the presence of a solution.
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.
Masha has 0 kopecks
Misha has 6 kopeks
The book costs 7 kopeks
Masha has .5 kopecks
Misha has 6.5 kopeks
The book costs 7.5 kopeks