You need to think about how the original price (578) is 100% of the price and that the sale price (78) is just a percentage of that. To discover how much of that 100% the price 78 is, you can write the formula:
578:100=78:x
X=(78*100)/578
X=14 %
Once you know that 78 represents the 14% of the whole price (100%-578) you just have to subtract that amount from the whole to understand which percent of discount has been applied: 100-14 = 86%
