Taken from here.
Say the Treasury issues an inflation-protected security with a $1,000 face value and a 3 percent coupon. In the first year, the investor receives $30 in two semiannual payments. That year, the CPI increases by 4 percent. As a result, the face value adjusts upward to $1,040.
In Year 2, the investor receives the same 3 percent coupon but this time it’s based on the new, adjusted face value of $1,040. The result: instead of receiving an interest payment of $30, the investor receives interest of $31.20 (.03 times $1,040). In Year 3, inflation drops to 2 percent. The face value rises from $1,040 to $1060.80, and the investor receives interest of $31.82.
I'm mostly confused about this line In Year 3, inflation drops to 2 percent. The face value rises from $1,040 to $1060.80, and the investor receives interest of $31.82.
Without knowing what the inflation rate was previous to "drop[ing] to 2 percent", how is the change in the principal of this TIPS calculated in this example?