Problem 15 from prior weeks.
Correct!
Explanation:
Solution: 4 comes next. This is the series of remainders when the positive perfect squares are divided by 9. 1 / 9 = 0 remainder 1; 4 / 9 = 0 remainder 4; 9 / 9 = 1 remainder 0; 16 / 9 = 1 remainder 7; etc. So the next number is the 11th positive perfect square's remainder when divided by 9: 121 / 9 = 13 remainder 4.