Identify the logic - Math Riddle

117.Math Riddles

Can you identify the logic behind the equation below and solve the puzzle by completing the equation? 
12 + 13 + 7 = 1, 15 + 16 + 11= 2, 17 + 18 + 16 = ?

B.4

Explanation :  

Logic Used: Sum the numbers and then subtract the two digits to get the desired answer.
12 + 13 + 7 => 32 => 3-2 => 1
15 + 16 + 11 =>42 => 4-2 => 2
17 + 18 + 16 = 51 => 5-1 => 4