Maximal elements are 10 and 20, minimal elements are 2 and 4, there is neither greatest element and nor least element.
→ The maximal element in the poset is an element that is divisible by all elements in poset is comparable to. so 10 and 20 are the maximal elements.
→The minimal element in the poset is an element that is divisible by all other elements in the relation they are comparable to. so 2 and 4 are minimal elements.
→There is no greatest element because 10 and 20 are incomparable which means that one doesn't precede the other.
→There is no least element because 2 and 4 are incomparable which means that one doesn't precede the other.