Andy estimated that he would need 78 feet of lumber for a tree house project. He later found that the actual amount of lumber needed was 67 feet. What was the percent error of Andy's estimation?

To calculate the percent error of Andy's estimation, you can use the formula:

\[ \text{Percent Error} = \frac{\text{Estimated Value} - \text{Actual Value}}{\text{Actual Value}} \times 100 \]

In this case:

• Estimated Value = 78 feet
• Actual Value = 67 feet

Now, plug in the values:

\[ \text{Percent Error} = \frac{78 - 67}{67} \times 100 \]

Calculate the difference:

\[ 78 - 67 = 11 \]

Now, divide by the actual value:

\[ \frac{11}{67} \approx 0.1642 \]

Finally, multiply by 100 to convert it to a percentage:

\[ 0.1642 \times 100 \approx 16.42% \]

Thus, the percent error of Andy's estimation is approximately 16.42%.

The correct answer is A. 16.42%.