math
posted by Mary .
If the cost of a cell phone has decreased 400% during the past 10 years, does that correspond to a decrease of four times?

To me, without more data, it doesn't make sense. A decrease of 100% would mean it costs nothing.
If the rate of decrease was constant over the 10 years, it would mean a decrease of 40% each year. Assuming it cost $100 originally:
$100
$60
$36
$21.60
$12.96
$7.78
That is only after 5 years!
Sorry that I cannot be of more help.