A. To calculate the total cost for Winona and Takona if they bought their mother the set of power tools, the expression would be 2d dollars.
B. The expression 0.90(d-5) refers to a discounted price for an item that originally cost d dollars. The 0.90 represents a 10% discount, and the (d-5) represents a $5 deduction from the original price. This expression could also be written as 0.90d - 4.50.
C. Let's choose d = $50.
For the power tools, the total cost would be 2 * $50 = $100.
For the water skis, the total cost would be ($50 + $15) = $65.
For the jewelry, the total cost would be ($50 - $15) = $35.
In this case, Winona and Takona should buy the jewelry as it is the least expensive option at only $35.