Put a number in each block so that each number not in the bottom row is the sum of exactly 2 blocks it touches (vertically or diagonally) below it.

1. | 2. | 3. | 4. | 5. |

6. | 7. | 8. | 9. |

10. | 11. | 12. | 13. |

14. | 15. | 16. | 17. |

18. | 19. | 20. | 21. |

The solutions are here.