The mean deviation of a normal curve is the average distance of each data point from the mean. In this case, the points of inflexion of the normal curve are given as 40 and 60.

Let's assume that the mean of the normal curve is μ.

To find the mean deviation, we need to calculate the average distance of each data point from the mean.

- Calculate the Mean of the Normal Curve:
- The mean of the normal curve can be found by taking the average of the inflexion points:
- μ = (40 + 60) / 2 = 100 / 2 = 50

- Calculate the Deviation of each Data Point from the Mean:
- The deviation of each data point from the mean can be calculated by subtracting the mean from each data point.
- Deviation of the first data point (40) = 40 - 50 = -10
- Deviation of the second data point (60) = 60 - 50 = 10

- Calculate the Absolute Value of Deviations:
- To find the distance of each data point from the mean, we take the absolute value of the deviations calculated in the previous step.
- Absolute deviation of the first data point = |-10| = 10
- Absolute deviation of the second data point = |10| = 10

- Calculate the Mean Deviation:
- The mean deviation is the average of the absolute deviations.
- Mean deviation = (10 + 10) / 2 = 20 / 2 = 10

Based on the above calculations, the mean deviation of the normal curve is 10, not 50. Therefore, the correct answer should be option 'A' (50) is incorrect.