You can solve this question using "rule of nine" to calculate the area of the body. Each arm equals to 4.5% so entire arms would be 4.5%.
Each side of body trunk equal to 18%, so entire back would be 18%.
Each leg equal to 18%, back of the leg would be half of it so it will be 9%.
The total would be: 4.5%+ 18%+9%= 31.5%