Approach is right.. some data is wrong as solving it is leading to irrational roots. To correct data, let n decreases by 5 and x increases by 5,
then we get 5 = n-x by your approach (instead of 3= n-x)
Next step is to put x= n-5 in the equation nx=300 , solve quadratic equation to get n = 20 and x =15
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Thanks Neha!!! I think on the actual test the problem was that I wasn't resubbing after the first substitution in order to get a quadratic and solve that way. I'm used to two variable/two equation problems being a little more straightforward. Thanks for clarifying.