please explain the answer

1) Miranda was reviewing the water bill for her dog day care and spa and determined that her highest bill, $5,000, occurred in July when she washed 500 dogs and her lowest bill, $3,600, occurred in February when she washed 325 dogs. What was the variable cost per dog associated with Miranda's water bill? 1) A) $8.00 3) Adam was a professional classical guitar player until his motorcycle accident that left him disabled. After long months of therapy,he hired an experienced luthier (maker of stringed instruments) and started a small shop to make and sell Spanish guitars. The guitars sell for $900 and the fixed monthly operating costs are as follows: Rent and utilities Wages and benefits to luthier3,000 Other expenses 800 500 Adam's accountant told him about contribution margin ratios and he understood clearly that for every dollar of sales, $0.60 went to cover his fixed costs, and that anything past that point was pure profit How many guitars does Adam have to sell each month to break even? 3) A) 8 guitars