The capacity of water heater is measured by its volume that they are capable of holding in. It may vary on your country's preference of measurement. In the United States, since they use US Metric System, they use gallons and the rest use liters.
Exact Form:
−3/10
Decimal Form:
−0.3
Sorry, very late response since I had other matters to attend to.
Question 1:
A: Incorrect; if that was the case, on day 1, the price would be 150. Instead here, according to the graph, on day 1 the price is 30.
B: Incorrect; the slope, or rate of change, is 30.
[Note: To calculate the slope, we can take two points (I've chosen points (1,30) [1] and (2,60) [2] for you) and use the formula

.

⇒

.

= x, therefore

= 30.]
C; Correct; each day the price increases by (+) 30.
D: Incorrect; the line does not show a constant decrease, but instead a constant increase.
Question 2:
A: Most likely this answer choice. The equation seems to be in slope-intercept form on accounting that the equation has mx and b. M has the x directly after it while the b is a number that stands alone. [Note: Numbers that stand alone and without a variable are constants.] B stands for the y-intercept/initial value, or amount started with.
In short:
Q1: C
Q2: A
If we assume the triangles are similar, then
.. 15/10 = x/8
.. x = 8*(15/10) = 12