C
the sum to infinity of a geometric series = 
where a is the first term and r the common ratio
= 
= 81 × 3 = 243
Answer:
-3 + -6= -9
because there is answer given
Answer:
Step-by-step explanation:
using form:
where a: starting number
where r: common ratio (in this case each next term is 3 times previous term)
The formula can be written in the form ...
... T = (original temperature) + x·(change in temperature each minute)
The problem statement gives you the original temperature (21 °C) and tells you the temperature after 12 minutes. You have to use that information to figure out the change in temperature each minute.
The change in temperature in 12 minutes is (75 °C) - (21 °C) = 54 °C. That is the change in 12 minutes, so the change in 1 minute will be 1/12 of that:
... 54/12 = 4.5
Using this value in the equation for T, we have
... T = 21 + 4.5x
I think that this is right, but I assumed that the ceiling has the same dimensions as the wall you would need 4 gallons for 2 coats.
Work:
20x8= 160 (square feet for 1 wall)
160x5=800 (i assumed there are 4 walls and 1 ceiling)
800x2=1600 (you need 2 coats)
1600/400=4 (each gallon includes 400 square feet)
