5. A triangle has sides of lengths $6 \mathrm{~cm}, 10 \mathrm{~cm}$ and $11 \mathrm{~cm}$. An equilateral triangle has the same perimeter. What is the length of the sides of the equilateral triangle?
A $18 \mathrm{~cm}$
B $11 \mathrm{~cm}$
C $10 \mathrm{~cm}$
D $9 \mathrm{~cm}$
E $6 \mathrm{~cm}$
5. D The perimeter of the equilateral triangle is $(6+10+11) \mathrm{cm}=27 \mathrm{~cm}$. Hence the length of the sides of the equilateral triangle is $27 \mathrm{~cm} \div 3=9 \mathrm{~cm}$.