Answer & Explanation
B) 64 %
Let the number be x.
Then, ideally he should have multiplied by x by 5/3. Hence Correct result was x * (5/3)= 5x/3.
By mistake he multiplied x by 3/5 . Hence the result with error = 3x/5
Then, error = (5x/3 - 3x/5) = 16x/15
Error % = (error/True vaue) * 100 = [(16/15) * x/(5/3) * x] * 100 = 64 %