Add. 4.29 + 97.2 + 0.687 =
102.177
140.787
1,407.87
1,021.77

4.29 + 97.2 + 0.687 = 102.177

In adding decimal numbers, make sure that the decimal points are aligned. Since each number has different counts of numbers after the decimal point, use 0 to pad the missing places.
    
      4.290
    97.200
      0.687
  102.177

The count of numbers after the decimal point is the same count of number of the decimal who has the greatest count of number after the decimal point.

4.29 only has 2 counts of places after the decimal point
97.2 only has 1 count of place after the decimal point
0.687 has 3 counts of places after the decimal point. 

The sum of the decimals must also have 3 counts of places after the decimal point.