The pair of numbers (6, 42) obeys a certain rule, and the pair (9, 90) obeys the very same rule. Which of these pairs obeys it too?

Look for what turns 6 into 42 and 9 into 90: $6\times 7=42$ and $9\times 10=90$, so the rule is 'multiply the number by the next number up'. Applying it to 5 gives $5\times 6=30$, so (5, 30) fits. The traps: (8, 64) squares the number instead of multiplying by the next one; (10, 90) multiplies by the number below rather than the number above, which is easy to slip into because 90 also appears in the given pair; (4, 24) multiplies by the number two higher, $4\times 6$.