I need a simple formula for Excel that solves the following problem: A coal company has two grades of coal ALMA and BUCK. What is the least expensive mix ratio (percent) of the two coals which will net less than 11% ash and 2.7% sulfur. Use a simple ratio by weight of the two components to calculate the final properties.ALMA is 12.5% ash, 2.2% sulfur, and costs $40.00 per metric ton

BUCK is 8.1% ash, 2.8% sulfur, and costs $50.00 per metric ton

I don't even know where to start with this one. I may in the wrong section of the forum, so any direction someone can give me would be appreciated.