Use Venn diagram: Let A = 42, B = 34, C = 20 Let overlaps: \[ A \cap B = 24, \quad B \cap C = 10, \quad A \cap C = 12, \quad A \cap B \cap C = 4 \] Apply:
\(\text{Total} = A + B + C - (A \cap B + B \cap C + A \cap C) + A \cap B \cap C = 42 + 34 + 20 - (24 + 10 + 12) + 4 = 96 - 46 + 4 = {54}\)
For any natural number $k$, let $a_k = 3^k$. The smallest natural number $m$ for which \[ (a_1)^1 \times (a_2)^2 \times \dots \times (a_{20})^{20} \;<\; a_{21} \times a_{22} \times \dots \times a_{20+m} \] is: