4. A small garden measuring $6 x 7$ meters has been divided into 5 square plots. All boundaries between the plots run parallel to the sides of the square, and the side of each plot is a whole number of meters. Find the total length of the resulting boundaries. Consider the boundaries as lines with no thickness.

ANSWER: $15 \mathrm{M}$.
Solution: 42 can be broken down into 5 squares in only one way: $42=4^{2}+3^{2}+3^{2}+2^{2}+2^{2}$. Their total perimeter is $4 * 4+4^{*} 3+4^{*} 3+4^{*} 2+4^{*} 2=56$. But we need to subtract the external boundaries, their length is $6+6+7+7=26$, and also take into account that each boundary participates in the perimeter of two squares - so we need to divide by 2.