Bigger triangular numbers are also possible, though you won’t see them at the bowling alley. After 10, the next one is 1 + 2 + 3 + 4 + 5 = 15, familiar from the setup at the start of a game of pool.

Triangular sums I have listed in random order five positive integers, four of two digits and one of one digit. They use each of the digits 1 to 9. None of the two-digit integers has any factor ...