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How can i solve to get the total price?

A suit is on sale for $400 and is on sale for 20% off. You present a coupon at the register for an additional 30% off. Use the decay factors to determine the price you pay for the suit
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2 Answers

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.)
Hi Olivia,
First the wording of this question is off, but I believe that I know what you meant. I will solve it in the manner that I believe is desired (with the suit being originally $400, not on sale for $400).
For the first discount of 20% you would multiply $400 X 0.8 = $320.
(Note: you could multiply by 0.2 which would give you the 20% off and then subtract that value from $400) 
(Thus $400 X 0.2 = $80 off and then $400 - $80 = $320. I just did a shorter step by doing the above.)
Then to address the additional 30% off you would take your new price ($320) and multiply it by 0.7 to find your total afterwards/
$320 X 0.7 = $224
(Note: my rational for using 0.7 can be made using the same argument as before)
Therefore your total comes to $224 after both discounts. 
Hope this helps!