Answer:
Frequently, we know the new amount and the percent change and need to know the original amount. We found that a $600 TV on sale for 30% off, cost $420 on sale. We might think, that to find the original cost of the TV that has a sale price of $420 after a sale of 30% off, we could just take 30% of the $420 cost and add it back to the $420 to find the original cost.
30% of $420 is 0.30($420) = $126 and $420 + $126 = $546.
Or, 130% of $420 is 1.3($420) = $546.
Notice that this is not the $600 original list price with which we started.
Explanation: