[ Divisibility of numbers. General properties ]

Yesterday, Nikita bought several pens: black ones for 9 rubles each and blue ones for 4 rubles each. Today, when he went to the same store, he found that the prices for the pens had changed: black pens now cost 4 rubles each, and blue pens cost 9 rubles each. Seeing this, Nikita said with disappointment: "If I had bought the same pens today, I would have saved 49 rubles." Is he mistaken?
The price of each pen has changed by 5 rubles, which means that the total cost of all pens has changed by a multiple of 5. Since 49 is not divisible by 5, Nikita is wrong.

## Answer

Nikita is wrong.