truthlover....The Insight book quotes Lange's Commentary on the Holy Scriptures in order to dismiss the LXX figures; Lange discusses in detail the artificiality of these numbers in comparison to the MT. Lange is certainly correct and most scholars I have read agree that the LXX numbers are not original. However the Insight book and the quote from Lange does not discuss the SP figures, nor do they critically examine the artificality in the MT figures. It is instead probable that the LXX, SP, and MT have each modified the figures in their own way, neither of which preserves the "original" figures in their entirety. It is quite clear however when one critically examines the evidence that the SP is most conservative, the LXX is most innovative, and the MT is somewhere in the middle.
There have been many attempts to reconstruct the original figures of ch. 5 of Genesis; the one I find most persuasive is that of Jacques Ruiten (I have made a few modifications of my own to his theory below). The SP is only slightly modified from the original, with the only figures changed being "rest of life" and "total age" for Jared, Methuselah, and Lamech:
Possible original form of the chronological data in Genesis 5
| age at first child | rest of life | total age |
Adam | 130 | 800 | 930 |
Seth | 105 | 807 | 912 |
Enosh | 90 | 815 | 905 |
Kenan | 70 | 840 | 910 |
Mahalalel | 65 | 830 | 895 |
Jared | 62 | 900 | 962 |
Enoch | 65 | 300 | 365 |
Methuselah | 67 | 902 | 969 |
Lamech | 53 | 700 | 753 |
Noah | 500 | 450 | 950 |
The Flood | 1,307 TOTAL | | |
Samaritan Pentateuch chronological data in Genesis 5
| age at first child | rest of life | total age |
Adam | 130 | 800 | 930 |
Seth | 105 | 807 | 912 |
Enosh | 90 | 815 | 905 |
Kenan | 70 | 840 | 910 |
Mahalalel | 65 | 830 | 895 |
Jared | 62 | 785 | 847 |
Enoch | 65 | 300 | 365 |
Methuselah | 67 | 653 | 720 |
Lamech | 53 | 600 | 653 |
Noah | 500 | 450 | 950 |
The Flood | 1,307 TOTAL | | |
The SP retains the original "age at first child" data and so the Flood occurs 1,307 years after the creation of Adam. But in these original data, Jared, Methuselah, and Lamech each outlive the Flood. So the SP artificially makes all three die in the same year -- the year of the Flood. The total of 1,307 years to the Flood is the basis for the ages in the "total age" column for three three patriarchs, and the figures in the middle column rest from the subtraction of the "age at first child". The derivation of the figures in the SP are thus pretty simple and straightforward.
LXX chronological data in Genesis 5
| age at first child | rest of life | total age |
Adam | 230 | 700 | 930 |
Seth | 205 | 707 | 912 |
Enosh | 190 | 715 | 905 |
Kenan | 170 | 740 | 910 |
Mahalalel | 165 | 730 | 895 |
Jared | 162 | 800 | 962 |
Enoch | 165 | 200 | 365 |
Methuselah | 167 | 802 | 969 |
Lamech | 188 | 565 | 753 |
Noah | 500 | 450 | 950 |
The Flood | 2,242 TOTAL | | |
The LXX figures are also rather straightforward. With the exception of Noah (whose figures are retained without change) and Lamech, the LXX adds 100 years to "age at first child" and subtracts 100 years from the "rest of life" figures. The "total age" figures are the same as the putative "original" figures. Lamech is the main exception. Instead of adding 100 years, 135 years are added to Lamech's "age at first child" and the same amount is subtracted from "rest of life". The extra 35 years resolves a different contradiction in the "original" figures. If Lamech was 153 years old at the birth of Noah, then Noah would have had his first son when Lamech was 653. Lamech died at age 753, which was 100 years later. That puts his death the year of the Flood, as Noah was 600 years old when the Flood came. But the "original" figures have Lamech die 1,407 years after the creation of Adam whereas if you total the "age at first son" figures, the Flood came 100 years earlier. But if you add 35 years to the age when Lamech became father to Noah, then he was 188 years old at Noah's birth and Noah had his first son when Lamech was 688 years old. Since Lamech died at age 753, he would have died 35 years before the Flood (i.e. 2,207 years after the creation of Adam) and 65 years after Noah became a father, and the contradiction is thereby avoided. The addition of the 35 years mirrors the 35 years spanning between the death of Lamech and the Flood. However because the "total age" for Methuselah is retained, Methuselah survives the Flood in the LXX.
The figures in the MT are the most complex:
MT chronological data in Genesis 5
| age at first child | rest of life | total age |
Adam | 130 | 800 | 930 |
Seth | 105 | 807 | 912 |
Enosh | 90 | 815 | 905 |
Kenan | 70 | 840 | 910 |
Mahalalel | 65 | 830 | 895 |
Jared | 162 | 800 | 962 |
Enoch | 65 | 300 | 365 |
Methuselah | 187 | 782 | 969 |
Lamech | 182 | 595 | 777 |
Noah | 500 | 450 | 950 |
The Flood | 1,656 TOTAL | | |
Notice first of all that the MT has exactly the same artificiality as the LXX in the case of Jared: the MT adds 100 years to "age at first child" and subtracts 100 years from "rest of life". The same is the case with Methuselah, except here the MT adds 120 years in the first column and subtracts 120 years in the second column. This addition of 20 years makes Methuselah die in the same year as the Flood. Finally, 129 years are added to Lamech's "age at first child" and 105 years are subtracted from the "rest of life" figure. The extra 29 years in the first column, when combined with the 20 years for Methuselah, yields a date for the Flood that matches the age of Methuselah at his death. Then 105 years are added to the second column to make Lamech die at the age of 777 -- an artificial number with possibly symbolic significance. This "total age" yields a date of death that is 5 years before the Flood -- the same 5 years that are added to 100 years subtracted from the age in the second column.
This analysis is attractive because it seems to account for most of the variances between the different versions.