How would you answer this (probably very simple) number-crunching question?

John and Susan are getting divorced after 10 years of marriage. It’s amicable and not very complicated – they don’t own real estate, they don’t have children.

The only complication is their retirement accounts. John, who is 45, has $150,000 socked away. Susan, who is 35, has $50,000 saved. All of it is money earned during their marriage. They live in a “community property” state, so a court would tell them to split evenly and each walk away with $100,000. But a court would also accept an agreement they make on their own.

So, John wants to explore another scenario. As he sees it, he is 10 years closer to retirement than Susan, so if they split 50/50, his savings won’t have as much time to grow as hers will. So he wants to propose some division where he takes a bit more than 50% and she takes a bit less, so that as each of them reaches retirement age (say, 65), they’d each have about the same amount to draw from.

How can John calculate a “fair” (as he sees it) division of the assets to propose? Clearly there will have to be some assumptions made (interest rates, inflation), but how can he figure out which slightly-unequal division today may lead to equal outcomes in the future?

Not asking whether John’s idea is stupid or unfair. Just wondering how to crunch the numbers. Never did understand accounting.

Explanations desired. Thanks in advance,

**E.**

Yes, it’s my divorce. Yes, we really are amicable and she knows I’m asking a bunch of internet strangers for money advice. Names, ages, and dollar amounts have been altered, not that it matters much.