Upon reading Jasher 29:17 and 18 you would never know that there was a mathematical error in those two  verses,  and the  reason  of this  study note is  because  I  want it to  be known  that I have  taken liberties so the calculations in my studies can continue without interruption and confusion.  Reading Jasher 29:17 & 18 we have this:

 17  And in those days Adah the wife of Esau conceived and bare him a son, and Esau called the name of the son that was born unto him Eliphaz, and Esau was sixty-five years old when she bare him.

  18  And Ishmael the son of Abraham died in those days, in the sixty-forth year of the life of Jacob, and all the days that Ishmael lived were one hundred and thirty-seven years and he died.

For Verse 17 we have to go further into chapter 29 and at verse 30 we read about Jacob fleeing for his life for Haran at the age of 77, but Esau learns of his brother’s departure and sends his son Eliphaz to kill Jacob.  At verse 33, Eliphaz’s age is shown to be at 13.  Subtracting 13 from 77, you get 64 which is not what what Verse 17 shows.  

Verse 18, we have to go back to Abraham when Ishmael was born, and we know that Abraham was 86 when Ishmael was born, but it wasn’t until Abraham was 100 when Isaac was born, then Isaac had to wait until he was 60 before Esau and Jacob were born, making Abraham 160, and Ishmael 74.  Therefore, if Ishmael was 137 years old when he died then Jacob would only be 63 years old and not 64 as verse 18 signifies.  Fortunately, these are the only discrepancies I have found in the Book of Jasher ... so far.