
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
Based on the following information, explore and complete the tasks:

Design a purchase and exchange plan.

- Material $1$: Xiao Ming tasted a delicious rice duck egg (green ball with pine pollen filling) at a classmate's house and inquired about its price. The classmate's mother said, "I forgot the specific price, but I only remember that the price of a rice duck egg is twice the price of a salty green ball. At that time, I bought both rice duck eggs and salty green balls. I bought 4 fewer rice duck eggs for $40 than the number of salty green balls I bought for $30."
- Material $2$: Xiao Ming's mother plans to spend $200 to buy two types of green balls for Xiao Ming and relatives. The quantity of both types of green balls is not less than 20, and the quantity of salty green balls is a multiple of 10.
- Material $3$: When Xiao Ming's mother paid $200 to buy green balls according to the plan in Material $2$, she received $m$ (where $1 < m < 10$) May Day promotional exchange coupons. After exchanging, the quantity of rice duck eggs is the same as the quantity of salty green balls.
  
Tasks to be solved:
1. Investigate the unit prices of the two types of green balls.
2. Explore the purchase plan.
3. Determine the exchange method.

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