4. A box with 30 identical balls has a mass of 650 grams. If we add 10 more such balls to the box, the total mass will be 800 grams. What is the mass of the box?
4. If adding 10 balls to the box increases the mass from $650 \mathrm{~g}$ to $800 \mathrm{~g}$, the mass of 10 balls is 
$800-650=150 \mathrm{~g}$. ..... 2 POINTS
Then the mass of 30 balls is $3 \cdot 150=450 \mathrm{~g}$. ..... 2 POINTS
This means the mass of the box is $650-450=200 \mathrm{~g}$. ..... 2 POINTS

TOTAL 6 POINTS