Rounding decimal numbers
Round to tenth, to hundredth and to thousandth the following decimal number: $$2,3851$$
To tenth $$\rightarrow 2,4$$
To hundredth $$\rightarrow 2,39$$
To thousandth $$\rightarrow 2,385$$
So, when rounding off, for example, to tenth, we write only one decimal. If the following decimal is bigger than $$0,05$$ we round it up (as in the previous case), otherwise we round it down. That is, $$2,38$$ would be $$2,4$$, while $$2,34$$ would be $$2,3$$.
Truncating decimal numbers
Truncate to tenth, to hundredth and to thousandth the following number: $$2,3851$$
To tenth $$\rightarrow 2,3$$
To hundredth $$\rightarrow 2,38$$
To thousandth $$\rightarrow 2,385$$
As we can see, to truncate to tenth it is enough to write the number with a decimal digit, to truncate to hundredth with two decimal digits and so on.