Add 1,197,481.24 + 421,496.81.

Step-by-step solution

1. 1

   Line up the decimal points, not the right-hand digits. Both numbers have exactly two decimal places, so the columns already match: hundredths under hundredths, tenths under tenths, units under units. This is the one alignment rule that decimal addition rests on.

2. 2

   Add the fractional parts first. $0.24 + 0.81 = 1.05$. The sum exceeds $1$, so the fractional digits are $.05$ and one whole unit carries into the integer column. Missing this carry is the usual source of a $1.00$ error.

3. 3

   Add the integer parts. $1{,}197{,}481 + 421{,}496$: units $1+6=7$, tens $8+9=17$ (carry $1$), hundreds $4+4+1=9$, thousands $7+1=8$, ten-thousands $9+2=11$ (carry $1$), hundred-thousands $1+4+1=6$, millions $1$. That gives $1{,}618{,}977$.

4. 4

   Bring in the carried unit. $1{,}618{,}977 + 1 = 1{,}618{,}978$, and appending the fractional digits gives $1{,}618{,}978.05.$

5. 5

   Verify with an exact fraction. In hundredths, $1{,}197{,}481.24 = \tfrac{119748124}{100}$ and $421{,}496.81 = \tfrac{42149681}{100}$; their sum is $\tfrac{161897805}{100} = \tfrac{32379561}{20} = 1{,}618{,}978.05$ exactly, with no rounding involved.

6. 6

   Estimate as a final sanity check. $1.197$ million $+\ 0.421$ million $\approx 1.618$ million, which agrees with the computed total to four significant figures.