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

First, calculate the error by subtracting the actual price from the estimated price. So the error is $4.50 - $4 = $.50. 
To find the percent error, divide the error by the actual price and multiply by 100 %. Hence, $.50/$4 =.125 x 100 = 12.5 %.
Dennis' percent error is 12.5 %.