(8.5 × 12 + 6.4 × 15 + 6 × 13) ÷ (12 + 15 + 13)
= (102 + 96 + 78) ÷ 40
= 276 ÷ 40
= 6.9 (元)
答:什锦糖每千克 6.9 元.