The Grenalat quarry sells small stones for garden decoration at a price of $ 1.50 per kilo and also backfill for house foundations at a price of $ 0.85 per kilo. These two types of stones come in different sizes and sell for a different price. To obtain stones of similar sizes, Grenalat passes them through a sieve which only lets through stones of the desired size. This device can sift a maximum of 200 kilos of garden stones in 40 minutes and 500 kilos of backfill in 1 hour 20 minutes To obtain the garden stones, variable costs of $ 0.65 per kilo are incurred, while the variable costs for backfill stones are $ 0.30 per kilo. Considering that Granelat customers cannot buy more than 2,100 kilograms of decoration stones and 2,100 of backfill stones per day, what would be the optimal combination of production of the two kinds of stones for an eight hour day?