Question 1008728
Call correct answers x and wrong answers y.  Then we have
x + y = 20  and
5x - 2y = 79
Now multiply the top equation by 5 and subtract it from the second equation and get
5x - 2y = 79
-(5x + 5y = 100)
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-7y = -21
y = 3 wrong answers leaving
x = 17 right answers