Look at this series: 21, 9, 21, 11, 21, 13,… What number should come next?

a. |
14 |

b. |
15 |

c. |
21 |

d. |
23 |

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Look at this series: 21, 9, 21, 11, 21, 13,… What number should come next?

a. |
14 |

b. |
15 |

c. |
21 |

d. |
23 |

Test ID: 541

Source: 501 challenging logic & reasoning problems

c. In this alternating repetition series, the random number 21 is interpolated every other number into an otherwise simple addition series that increases by 2, beginning with the number 9.