Chapter 5 - Linear Functions - 5-4 Point-Slope Form - Apply What You've Learned - Page 320: c

Using the equation from the previous question, $ y= 0.3x+6 $, the height of the water at 40 marbles would be, $y=0.3(40)+6 $, which is calculated to be 18. The function tells us that if 40 marbles were added in the glass, then the total level of the water would be 18 cm, and the total amount added to the level of the water by the marbles would be 12 cm.

Explanation above.