The sequence is: 3, 6, 9, 15, 24, … Isha thinks this is an "add 3 every time" sequence, since 3…

Why

Isha's rule correctly explains the first three numbers, which makes it feel confirmed — but a rule that works for three terms in a row can still be the wrong rule. Check the actual gaps here: 3→6 is +3, 6→9 is +3, but 9→15 is +6, and 15→24 is +9 — the amount being added is itself growing by 3 each time (+3, +3, +6, +9 — actually the real underlying pattern is each term is the sum of the previous two: 3+6=9, 6+9=15, 9+15=24). A short match is not proof; the real test is whether the SAME rule keeps working as the sequence continues. Test it: 24 + 15 = 39 would be next under the real rule, while '+3 forever' would wrongly predict 12. Her brother is right: the 'add 3' rule breaks down after the third term — checking only a few matching terms is not enough to trust a pattern.