When the number 42 is misread as 24, what is the percentage error?

Correct number = 42

Misread number = 24 

The formula to calculate percentage error is given by:

Percentage Error =$$\left( \frac{\text{True Value - Measured Value}}{ \text{True Value}}\right) × 100$$​​

In this case, the true value is 42 and the measured (misread) value is 24.
Substitute the values into the formula:
Percentage Error

​$$= \left[\frac{42 - 24}{42}\right] \times 100 \\\ \\= \left[\frac{18}{42}\right] \times 100 \\\ \\= 0.4286 \times 100 \\\ \\= 42.86\%$$​​
Therefore, the percentage error is 42.86%.