###Human: Elise is buying more dog food. She buys a 15kg bag then another 10kg bag, and she now has 40kg of dog food. How many kilograms of dog food did Elise already have?, ### Assistant: Elise has bought a total of 15 kg + 10 kg = 25kg of dog food.
This means she already had 40 kg – 25 kg = 15kg of dog food. 
 So the answer is :  15