By definition, a Decimal number is a number that has a whole part and a fractional part. Both parts are separated by a point called "Decimal point".
In this case, you have the following Decimal numbers given in the exercise:
[tex]1.6,2.01,1.63,1.51[/tex]You can identify that the Decimal number that has the greatest whole number is:
[tex]2.01[/tex]The others numbers have the same Whole number:
[tex]1[/tex]So, in order to identify their order from least to greatest, you need to see the digits after the Decimal point.
You can identify that:
[tex]0.51<0.6<0.63[/tex](Remember that "<" means "less than")
Then, knowing the above, you can determine that the order of the given numbers from least to greatest, is:
[tex]1.51,1.6,1.63,2.01[/tex]The answer is:
[tex]1.51,1.6,1.63,2.01[/tex]