This function calculates the Fill Rate Numerator per Barlow and Proschan description given an initial stock level (j), demand rate (Lam), and average time to restore (t).

1 |

`j` |
The initial stock level |

`Lam` |
The fail rate. |

`t` |
The average time to restore (repair time). |

Returns a scalar value for the FRN.

Richard E. Barlow and Frank Proschan, (1975) "Statistical Theory of Reliability and Life Testing, Probability Models"

1 | ```
FillRateNumerator<-FRN(5,.01,100)
``` |

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