The total price of the car is $20,000 but each year its price decrese by halve of the catual price, so the first year the new price will be:
[tex]\frac{20000}{2}=10000[/tex]so he reduce $10,000 dollar, but the next year it cost will decrease halve of the actula price so will be:
[tex]\frac{10000}{2}=5000[/tex]and is the same with a radiactive element that has 20,000 grams hi will reduce the same amounts, the only diference is that one reduces his mass and the other one reduces his value
