This is basically the PV of an ordinary annuity with a couple minor adjustments:
1) Since his payments are adjusted for inflation of 3% per year and the interest he earned is 6% per annum, we have to find the net of the two rates as follows:
1.06 / 1.03 =2.91262%. We also have adjust his first payment by 3% inflation rate as follows:
$50,000 / 1.03 =$48,543.69.
Now we can use the ordinary annuity formula to find the PV of all his 25 payments as follows:
PV=P{(1 + R)^N - 1]*[1 + R]^-N * R^-1}=PV OF $1 PER PERIOD.
PV=48,543.69{[(1 +0.0291262) ^25 - 1] * [1 + 0.0291262]^-25 * 0.0291262^-1}
=$853,587.68. This is PV of his annuity.
2)-
We can also sum up all his future payments, using summation formula, on any good calculator such as Wlofram/ Alpha engine as follows:
∑[50000*1.03^n / 1.06^(n+1)], n=0 to 24 =$853,587.68. And here is the link to W/A:
http://www.wolframalpha.com/input/?i=%E2%88%91%5B50000*1.03%5En+%2F+1.06%5E(n%2B1)%5D,+n%3D0+to+24