Erchin (Arachin) 4:4
Years are based on the one about whom the vow is made as follows: if a child dedicated the valuation of an elderly person, he pays the valuation of an elderly person; if an elderly person dedicated the valuation of a child, he pays the valuation of a child. Valuations are based on the one being evaluated as follows: if a man dedicated the valuation of a woman, he pays the valuation of a woman; if a woman dedicated the valuation of a man, she pays the valuation of a man. Valuation at the time of valuation is as follows: if one dedicated the valuation of a child under the age of five and he turns over five, or a person under 20 and he turns over 20, he pays what the valuation was at the time he made the obligation. The 30th day of life is the same as the days preceding it. The fifth year and the 20th year of life are the same as the years preceding them. This is derived from Leviticus 27:7, “If it will be from 60 years old and above….” Just as the 60th year of life is the same as the years preceding it, the same is true for the fifth year and the 20th years. But the 60th year being the same as the years preceding it is a stringency (in that one pays more for a 59-year-old than for a 60-year-old); can we assume the same of the fifth year and 20th year, in which being the same as the preceding years would create a leniency? (This is because one pays less for a four-year-old than for a five-year-old, and less for a 19-year-old than for a 20-year old.) We also rely on gezeira shava (repeated use) of the word shana (year): just as the shana of the 60th year is the same as the years preceding it, the same is true for the shana of the fifth and 20th years, whether this results in a leniency or a stringency. Rabbi Elazar says that the fifth, 20th and 60th years are the same as the preceding years until a person is a month and a day into that year (just as a newborn starts year one on his 31st day of life).
Erchin (Arachin) 5:1
If a person vows to donate his weight, he must pay his weight. If he committed to donate silver, he must pay in silver; if in gold, he must pay in gold. Yirmatya’s mother once vowed to donate her daughter’s weight; she went to Jerusalem, they weighed her daughter and she donated her weight in gold. Rabbi Yehuda says that if a person vows to donate the weight of his forearm, he fills a jar with water and sticks his arm in it up to his elbow. He then weighs the flesh, sinews and bones of a donkey until the volume refills the water he displaced from the jar. (The weight of these parts then equals the weight of his forearm.) Rabbi Yosi questioned how accurately one could weigh flesh against flesh and bone against bone. Rather, we merely estimate how much his forearm is likely to weigh.