Find the smallest possible set

Question

Find the smallest possible set (i.e., the set with the least number of elements) that contains the given sets as subsets. (Enter your answer using roster notation. Enter ∅ for the empty set.)

{1, 2}, {1, 3, 6}, {6, 9, 12, 15}

Answer 1

Easiest way is to combine the three sets, listing the elements in increasing order

Then cross out any repeated elements

So

(1,1,2,3,6,6,9,12,15)

Cross out a 1, and a 6 giving (1, 2, 3, 6, 9, 12, 15)