20%=20/100=1/5. Since there are no answers 1/5, we look for answers where the denominator is 5 times the numerator: 1/20 is 20*1 in the denominator, so that's not the answer; 2/20, 20=10*2; 4/20, 20=5*4, so 4/20 is the answer: 4/20=1/5=20/100=20%; 5/20, 20=4*5, 5/20=1/4=25/100=25%, so 5/20 is not the answer.