Does the subject want to ask "who is bigger, 4.2 or 3.9?" 4.2 big. According to the mathematical algorithm, 4.2 is greater than 3.9. 4.2 is 0.3 greater than 3.9. Method of comparing sizes:

1, compare the size of a decimal first, zd first looks at the left part of the decimal point and the left part of the decimal point, and the decimal point is larger; So is the left part.

2, first look at the integer part, the integer part is large, the integer part is the same, the decimal part is large, the decimal part is the same, the number in the percentile is the same, and so on.

3. To compare the sizes of two decimals, we should first compare whether their integer parts are the same. When the digits of the integers are the same, we should compare them from the high place.