The 114 represents 80% of the total price before the sale. You would set up your equation as
Cost Price * 80% = Sale Price.
Sale Price = 114
80% = 80/100 = 0.8
Cost Price = 114/0.8
Cost Price = 142.50 which is the price before the deduction. You can confirm this by taking 20% of 142.50 and subtracting it form 142.50
20% * 142.50
20/100 * 142.50
2850/100
28.50
Now subtract 28.50 from 142.50
142.50 - 28.50 = 114 which is just what you might expect.