In a class test, the mean of marks scored by a class of 40 students was calculated as 18.2. Later on, it was detected that the marks of one student was wrongly copied as 21 instead of 29. Find the correct mean.

$\text{ Mean of marks} = \dfrac{\text{Incorrect sum of marks}}{\text{No. of students}} \\[1em] \Rightarrow 18.2 = \dfrac{\text{Incorrect sum of marks}}{40} \\[1em] \Rightarrow \text{Incorrect sum of marks} = 40 \times 18.2 \\[1em] \Rightarrow \text{Incorrect sum of marks} = 728.$

As the marks of one student was wrongly copied as 21 instead of 29, correct sum of marks = 728 - 21 + 29 = 736.

$\therefore \text{Correct mean } = \dfrac{\text{Correct sum of marks}}{\text{No. of students}} \\[1em] = \dfrac{736}{40} = 18.4$

Hence, the correct mean is 18.4.