Half-lives of two radioactive elements A and B are 20 minutes and 40 minutes respectively. Initially, the samples have equal numbers of nuclei. After 80 minutes, the ratio of decayed numbers of A and B nuclei will be:

After 80min. for first element, number of nuclei left will be
N1=N(1/2)^(80/20)=N/16.
Similarly, for second element,
N2=N(1/2)^(80/40)=N/4
The number of decayed nuclei for A = N - N/16 = 15N/16
The number of decayed nuclei for B = N - N/4 = 3N/4
The ratio of decayed nuclei of A and B = (15N/16)/(3N/4) = (15N/16)*(4/3N) = 5/4 = 5:4