Let the no. of bugs he corrected be C and the no. of bugs that were defective be D.

So for every correct bugs he got 10000, so he got 10000C for correct bugs

Again for every defective bugs, he lose 5000,so he lose a total of 5000D.

Thus 10000C-5000D = 125000  —–i

and again C+D=20 ——ii

after solving these two equations we have C=15 and D=5

so he corrected 15 bugs and his 5 bugs detection were  defective