A. Winona would pay d dollars for the power tool set, so her expression would be d. Takona would also pay d dollars for the power tool set, so her expression would also be d. Therefore, the total amount they would pay is 2d.
B. The expression 0.90(d-5) refers to a discount of 10% off the original price (d-5) of an item. This can be determined because the expression is multiplied by 0.90, which is equivalent to multiplying by (1-0.10). The expression could also be written as 0.10(d-5) or 0.1d - 0.5.
C. Let's choose d = 50$.
For the power tools, Winona and Takona would each pay 50$.
For the water skis, Winona would pay 50$ + 15$ = 65$ and Takona would pay 50$ + 15$ = 65$.
For the jewelry, Winona would pay 50$ - 15$ = 35$ and Takona would pay 50$ - 15$ = 35$.
Based on the given prices, the power tools would be the most cost-effective present for both Winona and Takona. Therefore, they should buy the power tool set for their mother.