
When tackling complex reasoning tasks, you have access to the following actions. Use them as needed to progress through your thought process.

[ASSESS]

[ADVANCE]

[VERIFY]

[SIMPLIFY]

[SYNTHESIZE]

[PIVOT]

[OUTPUT]

You should strictly follow the format below:

[ACTION NAME]

# Your action step 1

# Your action step 2

# Your action step 3

...

Next action: [NEXT ACTION NAME]


system
To protect the environment while also making rational use of natural resources, a high-end sanatorium was built at the foot of Wei Bao Mountain. Each month, every resident is offered two water supply plans:  
Plan 1: Supply of tap water from Wei Bao Mountain's reservoir, with a water fee of 2 yuan per ton;  
Plan 2: Limited supply of up to 10 tons of Wei Bao Mountain's mineral spring water.  
In Plan 2, if the usage of spring water does not exceed 5 tons, it is charged at a basic price of 8 yuan per ton. For the part exceeding 5 tons but not exceeding 8 tons, it is charged at 1.5 times the basic price. For the part exceeding 8 tons, it is charged at 2 times the basic price.  
(Ⅰ) Please write the function relationship between the spring water usage fee $y$ (yuan) and its usage volume $x$ (tons);  
(Ⅱ) When Mr. Wang, a resident, was paying the fees for December, he was informed that he used a total of 16 tons of water and was charged 72 yuan. How many tons of tap water and spring water did he use in that month?

Present the answer in LaTex format: \boxed{Your answer}
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