A warranty is written on a product worth $10,000 so that the
buyer is given $8,000 if it fails in the first year, $6,000 
if it fails in the second, $4,000 if it fails in the third, 
$2,000 if it fails in the fourth, and $0 after that. 

The probability of the product's failing in a year is 0.1; 
failures are independent of those of other years. 
What is the expected value of the warranty?