A student erroneously multiplied a number by 2/5 instead of 5/2.What is the percentage error in the calculation?

Sure, here is the step by step solution:

1. Let's assume the original number to be x.

2. The student should have multiplied x by 5/2, so the correct result should be (5/2)x.

3. But the student multiplied x by 2/5, so the erroneous result is (2/5)x.

4. The error in calculation is the difference between the correct result and the erroneous result, which is (5/2)x - (2/5)x.

5. To find the percentage error, we divide the error by the correct result and multiply by 100. So, the percentage error is [((5/2)x - (2/5)x) / (5/2)x] * 100.

6. Simplifying this, we get the percentage error as 140%.

So, the student made a 140% error in the calculation.