Material Mix

Hello, I have the following questionL Product GX consists of a mix of three materials, J, K and L. The standard material cost of a unit of GX is as follows: $ Material J 5 kg at $4 per kg 20 Material K 2 kg at $12 per kg 24 Material L 3 kg at $8 per kg 24 During March, 3,000 units of GX were produced, and actual usage was: Material J 13,200 kg Material K 6,500 kg Material L 9,300 kg. In the answers it states the following: 3,000 units should use 10 kg each (3,000 × 10) = 30,000 kg 3,000 units did use = 29,000 kg Difference = 1,000 kg favourable Valued at $6.80 per kg ($68/10 kg) Variance = $6,800 favourable So my question is - how exactly do we calculate the 6.80 - where does the 68 come from? Thank you in advance