Half-life is a measure of the rate at which a radioactive substance undergoes decay. It represents the time it takes for half of the radioactive nuclei in a sample to decay. In this question, we are given two radioactive elements A and B with half-lives of 20 minutes and 40 minutes respectively.

To solve this problem, let's consider the decay of each element separately and then compare the ratios.

Decay of Element A:
- After 20 minutes, half of the nuclei of A will decay, and we will be left with half of the initial number of nuclei.
- After another 20 minutes (40 minutes in total), half of the remaining nuclei will decay, leaving us with a quarter of the initial number of nuclei.
- After another 20 minutes (60 minutes in total), half of the remaining nuclei will decay, leaving us with an eighth of the initial number of nuclei.
- After another 20 minutes (80 minutes in total), half of the remaining nuclei will decay, leaving us with a sixteenth of the initial number of nuclei.

Decay of Element B:
- After 40 minutes, half of the nuclei of B will decay, and we will be left with half of the initial number of nuclei.
- After another 40 minutes (80 minutes in total), half of the remaining nuclei will decay, leaving us with a quarter of the initial number of nuclei.

Comparing the ratios:
- After 80 minutes, the ratio of decayed numbers of A and B can be calculated by dividing the number of decayed nuclei of A (1/16) by the number of decayed nuclei of B (1/4).
- Simplifying this ratio, we get 1/16 divided by 1/4, which is equal to 1/16 multiplied by 4/1.
- Thus, the ratio of decayed numbers of A and B is 1/64.

Since the question asks for the ratio of decayed numbers of A and B, we need to simplify this ratio further:
- 1/64 can be written as 1:64.
- We can further simplify this ratio by dividing both sides by 4, giving us the final ratio of 1:16.

Therefore, the correct answer is option C, 5:4.