if you look closely, 3/6 is equivalent to 1/2, so from that we can infer that the question is asking for 1/2 equivalents; for example, 9 is half of 18 and 20 is half of 40.
Set up a proportion. take 3/6 = 20/x Cross multiply 3 * x = 6 * 20 Simplify 3x = 120 Divide by 3 x = 40 So the denominator of the bottom fraction is 40