
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
In April 2020, the "One Helmet One Belt" safety protection campaign was actively carried out nationwide. The sales of a certain brand of helmets have been increasing month by month. It is known that the sales in April were 150 units, and in June they were 216 units, with the monthly growth rate of sales from April to June being the same.
$(1)$ Find the monthly average growth rate of sales for this brand of helmets;
$(2)$ If the cost price of this helmet is $30$ yuan per unit, and it was found that when the selling price is $40$ yuan per unit, the monthly sales volume is 300 units. For every increase of $1$ yuan in selling price, the monthly sales volume decreases by 10 units. In order to achieve a monthly sales profit of $3960$ yuan and provide customers with the best value, what should be the selling price of this brand of helmets per unit?

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