A. The expression to show how much Winona and Takota would pay in total if they buy their mother the set of power tools is: D - (0.15 * D). Simplifying, this becomes: 0.85D.
B. The expression 0.90 (D - 5) refers to the total paid for the jewelry after a 10% discount. We know this because the expression is multiplied by 0.90, which represents a 10% discount. This expression could also be written as: 0.90D - 4.50.
C. Let's choose a value for D as $50.
For the power tools, the total cost would be: 0.85 * 50 = $42.50.
For the water skis, the total cost would be: (0.85 * 50) + 15 = $57.50.
For the jewelry, the total cost would be: (0.90 * 50) - 5 = $40.
Based on the chosen value of D, it would be best for Winona and Takota to buy their mother the jewelry. It has the lowest total cost at $40. Additionally, the 10% discount makes it even more affordable.