1. For a gift to his son, the father bought 4 books. All books except the first cost 84 k., except the second - 80 k., except the third - 76 k., and except the fourth - 72 k. What is the cost of each book?
1. If we add four numbers $84, 80, 76, 72$, we get the tripled cost of all books, so the cost of the books is: $(84+80+76+72): \dot{3}=$ $=312: 3=104$ (k.). Therefore, the first book costs $104-84=20$ (k.), the second $104-80=24$ (k.), the third $104-76=28$ (k.), and the fourth $104-72=32$ (k.).