24. The diagram shows a map of a park. The park is divided into regions. The number inside each region gives its perimeter, in $\mathrm{km}$. What is the outer perimeter of the park?
A $22 \mathrm{~km}$
B $26 \mathrm{~km}$
C $28 \mathrm{~km}$
D $32 \mathrm{~km}$
E $34 \mathrm{~km}$
SOLUTION
B

The sum of the perimeters of $F, G, H, I$ and $J$ give the length of the outside line increased by the dotted line. If we subtract the perimeters of $K, L$ and $M$, then we subtract the dotted line but we have now also subtracted the dashed line. So we add the dashed line to compensate. In other words the required perimeter is $(F+G+H+I+J)-(K+L+M)+N$. Here it is $42-20+4$, that is $26 \mathrm{~km}$.