Dennis wants to buy a card for his wife. dennis calculates the amount of the card as $4.50, the actual price of card is $4. what is dennis'w percent error

The problem has quiet a few information's that need to be checked first. These information's will help to determine the required answer.
Actual price of the card = $4
Price of the card calculated by Dennis = $4.50
Amount of error made by Dennis while calculating
the cost of the card = (4.50 - 4) dollars
                               = 0.50 dollars
Percentage of error made by Dennis while
calculating the cost of the card = (0.50/4) * 100
                                                  = 0.50 * 25
                                                  = 12.5 percent
So 12.5% error has been made by Dennis while calculating the price of the card that he wants to buy for his wife.