Farah compares two numbers, 4,809 and 4,890, and says 4,809 is bigger because it ends in 9, and 9…

Why

Farah's instinct that 9 is important is not wrong — 9 really is the biggest single digit. Where it breaks is treating "has a 9 somewhere" as the whole comparison, when WHERE a digit sits matters far more than what the digit is. Compare 4,809 and 4,890 place by place from the left: both have 4 in the thousands and 8 in the hundreds, so those cancel out — the decision comes down to the tens place, where 4,890 has a 9 and 4,809 has a 0. That single place makes 4,890 the bigger number, 4,890 > 4,809, even though 4,809 has a 9 sitting in the ones place. A ₹100 note and ten ₹10 notes both "have a 9" if you write out ₹190 versus ₹109 — but which pile of money you'd rather hold depends on where that 9 sits, not that it exists. Her brother is right: 4,890 is the bigger number, and comparing left to right, one place at a time, is what decides it.