Clue | Number |

Triangle number before 21 | |

Half of second-lowest perfect number | |

Lowest non-palindromic number | |

Neither prime nor composite | |

Sum of first 6 positive integers | |

Lowest perfect number | |

Number of Platonic solids | |

Fibonacci number before 21 | |

Only even prime | |

Smallest number that cannot be expressed as the sum of eight or fewer perfect cubes | |

x + x = x * x = this number | |

Second lowest number less than the sum of its factors (excluding itself) | |

Neither prime nor composite | |

Only even prime | |

Number of points needed to determine a plane | |

x + x = x * x = this number | |

Number of Platonic solids | |

Lowest perfect number | |

Smallest prime factor of 91 | |

Smallest even perfect cube | |

Highest power of 3 one greater than a power of 2 | |

Lowest non-palindromic number | |

Number of sides in an undecagon | |

Number of ways to rearrange the letters in 'THAT' | |

Fibonacci number before 21 | |

Half of second-lowest perfect number | |

Triangle number before 21 | |

Lowest two-digit power of two | |

The lowest number that is the sum of a positive perfect cube and a positive perfect square in two ways | |

Second lowest number less than the sum of its factors (excluding itself) | |

Maximum number of unit cubes visible at one time in a 3x3x3 cube | |

Multiplicative inverse of 0.05 | |

Sum of first 6 positive integers | |

Number of possible partitions of a circle that can be made with 6 lines | |

Smallest number that cannot be expressed as the sum of eight or fewer perfect cubes | |

Factorial of 4 | |

Lowest perfect square that is the sum of two other perfect squares | |