6.4. The frame for three square photographs has the same width everywhere (see figure). The perimeter of one opening is 60 cm, the perimeter of the entire frame is 180 cm. What is the width of the frame?
Answer: 5 cm.

Solution. From the condition of the problem, it follows that the length of one hole's side is 15 cm. Let $d$ cm be the width of the frame, then the perimeter of the rectangle is $8 \cdot 15 + 12 d = 120 + 12 d$ (cm). According to the condition, $120 + 12 d = 180$, which means $d = 5$.

Similar reasoning can be conducted without setting up an equation.

## Evaluation Criteria:

+ fully justified solution

$\pm$ the equation is correctly set up (correct line of reasoning), but an arithmetic error is made in solving the equation (in performing a certain operation)

干 the correct answer is guessed and verified that it satisfies the condition

- only the answer is provided
- the problem is not solved or is solved incorrectly