7. Mr. Sheng is using a faucet attachment to fill the fish tank in his biology classroom. The graph below shows the amount of water in the tank during the first several minutes since he turned on the faucet. What is the slope of a line joining the points on the graph? If your answer is not an integer, express it as a decimal.
Filling a Fish Tank
Volume (gallons)
55
50
45
40
35
30
25
20
15
10
5
(216)
.
二
1
2 3 4 5 6 7 8
Time (minutes)