Answer:
The rate of decay each month is -0.006924%
Step-by-step explanation:
The cost of a car after 1 year at a 8% decreased
Decreased price = Sale price - (Sale price x porcentage of decreased/100%)
Decreased price = $20,000 - ($20,000 x 8%/100%)
Decreased price = $20,000 - ($20,000 x 0.08)
Decreased price = $20,000 - $1,600 = $18,400
So, the value of the car decreased by 8% in a year is $18,400.
The general equation for exponential decay is:

Where
y = final amount
C = Sale price
r = rate of decay
t = time
We know that the sale price was $20,000. After a year the decreased price is $18,400.
From the general equation for exponential decay, our sale price is $20,000 decreased over a time of 12 months resulting the final amount of $18,400.
substituting the values

Solving the equation for r


![\frac{18,400}{20,000}^{\frac{1}{12} } =[(1-r)^{12}]^{\frac{1}{12}}](https://tex.z-dn.net/?f=%5Cfrac%7B18%2C400%7D%7B20%2C000%7D%5E%7B%5Cfrac%7B1%7D%7B12%7D%20%7D%20%3D%5B%281-r%29%5E%7B12%7D%5D%5E%7B%5Cfrac%7B1%7D%7B12%7D%7D)





![r=1-\sqrt[12]{\frac{18,400}{20,000}}](https://tex.z-dn.net/?f=r%3D1-%5Csqrt%5B12%5D%7B%5Cfrac%7B18%2C400%7D%7B20%2C000%7D%7D)
![r=1-\sqrt[12]{0.92}\\r=1-0.993076\\r= -0.006924](https://tex.z-dn.net/?f=r%3D1-%5Csqrt%5B12%5D%7B0.92%7D%5C%5Cr%3D1-0.993076%5C%5Cr%3D%20-0.006924)