Parents at a school in Ajmer attended Sports Day, Annual Day or Open House, as the diagram shows. How many parents attended at least one of the three events?

Add each part once: $14+22+17+6+5+8+3=75$. 72 forgets the 3 in the centre, 53 counts only the parents who attended exactly one event, and 100 adds the three circle totals, which counts the overlaps more than once.