In a Dobereiner’s triad, the respective atomic masses of the first and the last elements are 40 and 137
What is the mass of the middle element of the triad?
If A and B are 2 sets such that A U B has 40 elements, A has 18 elements and B has 29 elements, how many elements does A ∩ B have? __
If A and B are 2 sets such that A ∪ B has 40 elements, A has 18 elements and B has 29 elements, how many elements does A ∩ B have?