In Adam's expression, d represents the original price of the game. 0.75d represents 75% of the original price; this is the amount of the discount, since everything is 75% off. Taking the difference of the original price and the discount, d-0.75d, gives us the total price of the game. In Rena's expression, 0.25 represents 25%. This is because taking 75% off of the price means we still pay 100-75 = 25% of the original price. Multiplying the original price, d, by the 0.25, gives us 25% of the original price; this is the total price.
