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From combined age of 137 years first subtract 70 years and then subtract 31 year.
Then divide the result by 3 to get Jim's age as 12 years.
Then the father's age is 12 + 31 = 43 and the grandfather's age is 12 + 70 = 82 years.
CHECK. 12 + 43 + 82 = 137. ! Correct !
Solved.