I've never heard of a decay factor method. I looked it up and everything I saw seemed to relate it as a decay over a period of time, like the opposite of an exponential growth. I saw nothing related to a question like this. However, since what I saw seems to be essentially the proportion of what you'd have after you got done "decaying," the following method I know might be appropriate:
If we assume Emily is correct that $400 is the original price, you can do this as one percent which is sometimes taught in business math classes. It still uses the 80% and the 70%, but it's a way of finding one percent you can use rather than doing them one at a time.
You would take .80 x .70 = .56. (56%)
I know this as a "cost equivalent," because you pay the equivalent of 56% of the original price.
You can then just multiply 400 x .56.
(The nice thing about this method is that once you have the 56%, you can use it on any original cost without doing two multiplications.)