Notice how 1/2 and 3/8 are positive numbers. We add two positive numbers meaning that the result should be larger. However 2/5 = 0.4 is not larger than 1/2 = 0.5
So something went wrong with the adding. It turns out that the error is when the person added the numbers in the denominators (2 and 8 to get 10). This is not allowed. The denominators must be the same and stay the same in order for the fractions to be added.
For example: 1/7 + 2/7 = (1+2)/7 = 3/7
The 7s in the denominator do not change
Therefore the answer is choice B) Not true because 2/5 < 1/2