a) The equation 
represents the discharge of the Grand Prismatic Spring.
b) The discharge of the Grand Prismatic Spring takes 15 minutes.
c) The remaining quantity after 7 minutes is 4480 gallons.
d) The hardness of cardboard is 2.
<h3>How to apply concepts of linear functions</h3>
a) In this case, the volume of water (
), in gallons, decreases <em>linearly</em> in time (
), in minutes. Then, we could model the situation by using the following expression:
(1)
Where:
- Initial volume, in gallons.
- Discharge rate, in gallons per minute.
If we know that 
and 
, then the volume of the Grand Prismatic Spring is represented by this expression:
(2)
The equation represents the discharge of the Grand Prismatic Spring. 
b) By (1) and 
we find the time required for discharge:
The discharge of the Grand Prismatic Spring takes 15 minutes.
c) The quantity of gallons remaining is found by evaluating the function for 
:
The remaining quantity after 7 minutes is 4480 gallons.
d) According to Mohs scale, the hardness of feldspar is 6. The statement indicates the following relationship, as there is a constant relationship between both hardnesses:
(3)
Where:
- Hardness of cardboard.
- Hardness of feldspar.
If we know that 
, then the hardness of cardboard is:
The hardness of cardboard is 2.
To learn more on linear functions, we kindly invite to check this verified question: brainly.com/question/17058347
