Photo frame

A square shape photo with a side length of 20 cm is framed by a 4 cm wide bar. Find the external frame size of this photo (the frame is from all sides)

Correct answer:

b =  28 cm

Step-by-step explanation:

$$\begin{gathered} a=20\text{ m } \\ r=4\text{ cm } \\ {S}_1=a\cdot a \\ {S}_2=b\cdot b \\ b=a+2\cdot r=20+2\cdot 4=28\text{ cm} \end{gathered}$$

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