I got $678,158.
I don’t have the curriculum, and I don’t have the SchweserNotes with me (I’m out of town teaching), so I don’t know what the formula is, but that doesn’t matter: I wouldn’t bother trying to remember another stupid formula. I simply think through the steps: what happens to the money? I’ll go through them.
We’re earning 7.5% per year, half of which is taxed at 10%. That’s an effective tax rate of ½ × 10% = 5%, so, net of taxes, we’re earning 7.5% × (100% - 5%) = 7.5% × 95% = 7.125% per year, compounded.
Thus, our ending account balance (before we close the account and pay the rest of the taxes) is:
$250,000 × (1.07125)^15 = $701,944.
That gives us a gain of $701,944 – $250,000 = $451,944.
Now comes the tricky bit: 45 parts of that gain represent the (previously) realized (and taxed) portion, and 50 parts represent the (previously) unrealized (and untaxed) portion. The way I got 45 and 50 parts is this:
Realized (and taxed) return: ½ × 90% = 45% (of the pre-tax return). (We got to keep 90% of the half that got taxed)
Unrealized (and untaxed) return: ½ × 100% = 50% (of the pre-tax return). (We got to keep 100% of the half that didn’t get taxed.)
Total return: 45% + 50% = 95% (of the pre-tax return).
So 45/95 has already been taxed, 50/95 hasn’t.
50/95 × $451,944 = $237,865.
The tax on $237,865 (at 10%) is $23,787.
Finally, after paying that tax, we’re left with $701,944 – $23,787 = $678,158 (except for rounding).
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Let’s look at another example of tax splits, so you can see how to handle them; this will help make clear the 45, 50, and 95 parts above.
Suppose that ¾ was realized and ¼ unrealized, and the tax on the realized portion was 20%; the tax on the unrealized portion will be 30%. Your annual, after-tax return would be (0.75 × 80% + 0.25) × 7.5% = 0.85 × 7.5% = 6.375%
After 15 years you’d have $250,000 × (1.06375)^15 = $631,733.
The total gain is $631,733 – $250,000 = $381,733.
Taxed portion: 0.75 × 80% = 60% (of the pre-tax return).
Untaxed portion: 0.25 × 100% = 25% (of the pre-tax return).
Total return = 60% + 25% = 85% (of the pretax return).
Untaxed gain = 25/85 × $381,733 = $112,274.
Tax = 30% × $112,274 = $33,682.
Final balance = $631,733 – 33,682 = $598,051.
I hope that this helps.