What is the relationship between two elements X and Y whose atomic number is 18 and 20 respectively, but their mass numbers remain same as 40?

The elements are Isobars.

Explanation:

If two elements have different atomic numbers, but same atomic mass they are called isobars. Here, X and Y have atomic number 18 and 20, but have the same atomic mass. So, they are clearly isobars. They have differing atomic numbers because the no. of protons in each element is different. But, they end up having the same atomic mass because they have enough neutrons to equate the sum total of nucleons between them. Isobars are present in different regions of the periodic table.