Put equal amounts of that 15% into just three different mutual funds:

• A U.S. total stock market index fund
• An international total stock market index fund
• A U.S. total bond market index fund

Over time, the three funds will grow at different rates, so once per year
you'll adjust their amounts so that they're again equal.

That's it.

Vanguard Total Stock Market Index, MUTF: VTSMX
Vanguard Total International Stock Index Fund Investor Shares, MUTF: VGTSX
Vanguard Total Bond Market Index Fund Investor Shares, MUTF: VBMFX

annual salary   total contributions    final investment
rise (%)        over 21 years          value after 21 years
  0               20,955                    43,557
  1               23,210                    47,265
  2               25,776                    51,424

funds = {"VTSMX", "VGTSX", "VBMFX"};

{tsm, ism, tbm} = FinancialData[#, {"April 29, 1996", DateList[], "Month"}] & /@ funds;
DateListPlot[{tsm, ism, tbm}, PlotLegends -> funds]

salary = 550;
investment = salary*0.15;
nmonths = Length[tsm] - 1;
d = ConstantArray[investment/3, nmonths];

(* add in inflation if wanted *)
inflation = 0.02;
y = First /@ Partition[d, UpTo@12];
iy = Array[y[[#]] (1 + inflation)^(# - 1) &, Length[y]];
d = Take[Flatten[ConstantArray[#, 12] & /@ iy], nmonths];

DateListPlot[Transpose[{Rest[First /@ tsm], 3 d}],
 PlotLabel -> Row[{"Monthly contributions - Total contributed: ",
    Total[3 d]}], PlotRange -> {Automatic, {0, Automatic}},
 PlotMarkers -> {Automatic, 6}]

{tsm2, ism2, tbm2} = Ratios@# - 1 & /@ Map[Last, {tsm, ism, tbm}, {2}];
d2 = 0;
vals = Array[(d2 = (d[[#]] + d2/3) (tsm2[[#]] + ism2[[#]] + tbm2[[#]] + 3)) &, nmonths];
DateListPlot[Transpose[{Rest[First /@ tsm], vals}], PlotMarkers -> {Automatic, 8},
 PlotLabel -> Row[{"Investment value over time - Final value: ", Last[vals]}]]