How do I find the formula to explain the blended APY 5 APY u

How do I find the formula to explain the blended APY?

5% APY up to $5000.
Earns at 0.39% at $5001 and above.
The blended APY is 0.61%.

The blended APy is calculated as weighted average APY earned

Blended Apy = (Amount1 * Rate1 + Amount2 * Rate2) / Total Amount,

in the given condition of in question ,

= (5000*0.05 +1* 0.0039) / (5000+1)

= 0.049991 =4.999%

Note :The answer seems to be little discomfortable .Kindly check the data in the question , as answer is true for given data