If 87 + 92 = 18; 57 + 83 =13; 12 + 61 = 4 then 62 + 31 = ?

Question

If in a coded language
1.67 * 2.31 = 129
2.39 * 4.31 = 202
6.78 * 4.87 = 816
Then
7.16 * 9.13 = ?

Answer 1

87 + 92 = 18; ((8+7)+(9-2)-4)
57 + 83 =13; ((5+7)+(8-3)-4)
12 + 61 = 4; ((1+2)+(6-1)-4)
therefore
62 + 31 = ? ((6+2)+(3-1)-4) = 6