I don't think you need to do anything special for this question. But do be careful how you read it.
Total number of instruments = 63
Number of woodwinds = 21
Number of non woodwinds = 63 - 21 = 42 non woodwinds.
What you have to be careful about is that this is not the answer.
It says how many more .... So now you have to do another subtraction.
Total number more = Number of non woodwinds - woodwinds.
Total number more = 42 - 21
Total number more = 21 <<<< Answer
