Problem 1. A box with 30 identical marbles has a mass of 650 grams. If we add 10 more such marbles to the box, the total mass will be 800 grams. What is the mass of the box?
Solution. With the addition of 10 balls to the box, the total mass increases by $800-650=150$ grams. According to this, one ball has a mass of $150: 10=15$ grams. Therefore, all the balls in the box have a mass of $30 \cdot 15=450$ grams. Thus, the mass of the box is $650-450=200$ grams.