First, you would put 4 as the numerator, because the silk butterfly's wingspan is 4 cm. Then, you would put 20 as the denominator, because the butterfly's wingspan is 20 cm. Now we have 4/20, which simplifies to 1/5 (the GCF of 4 and 20 is 4, so 4 divided by 4 = 1 and 20 divided by 4 = 5). Therefore, you answer would be 1/5.