【Answer】Solution: $15+16+18+19+20+31=119$ (kg), the total amount of food bought by the canteen is: $119-20=99$ (kg), $99 \div 3=33$ (kg),

The weight of the food bought the second time is: $15+18=33$ (kg), the weight of the food bought the first time is: $16+31+19=66$ (kg); Answer: The weight of the remaining bag is 20 kg.
Therefore, the answer is: 20 .
7. (6 points) In the grain store, there are 6 bags of flour, weighing $15, 16, 18, 19, 20, 31$ kilograms respectively. The canteen bought 5 of these bags in two purchases. It is known that the weight of the first purchase is twice that of the second purchase. The weight of the remaining bag is $\qquad$ kilograms.