See, my answer is in the form of a maths trick, the answer may be wrong because I has little casual in doing the answer.
There is a maths formula to calculate the Net Increase or Decrease percentage :-
Net %age increase or decrease = a + b +  \frac{ab}{100}

We can use the above formula directly if values of a and b can be are in the form of multiplication in the question. In the question, let number of days be one value and number of wages PER DAY be another value. We get to know that the net wage will be in the form of multiplication of the above values. 

So, by using the formula we can write that :-
20-15- \frac{300}{100}

= 2%