Bài 3 :
$5,(6)=5+\frac{2}{3}=\frac{17}{3}$
$0,(15)=\frac{15}{99}$
$1,(36)=1+\frac{4}{11}=\frac{15}{11}$
$0,21(3)=0,21+\frac{1}{300}=\frac{21}{100}+\frac{1}{300}=\frac{16}{75}$
$7,1(18)=7+\frac{13}{110}=\frac{783}{110}$
$0,01(6)=\frac{1}{60}$

Bài 4 :
$2,(4).\frac{3}{11}=\frac{22}{9}.\frac{3}{11}=\frac{2}{3}$
$-0,(3)+\frac{1}{3}=-\frac{1}{3}+\frac{1}{3}=0$

Bài 6 :
Số thâp phân hữu hạn $a,bc=a+\frac{\overline{bc}}{100}=\frac{100a+10b+c}{100}$
$a,bc=\frac{a+b+c}{4}$
$\Leftrightarrow \frac{100a+10b+c}{100}=\frac{a+b+c}{4}$
$\Leftrightarrow 100a+10b+c=25(a+b+c)$
Vì $25\vdots 5\Rightarrow 25(a+b+c)\vdots 5\Rightarrow (100a+10b+c)\vdots 5$
$\Rightarrow c=5$ hoặc $c=0$( loại)
$\Rightarrow c=5$
$\Rightarrow 100a+10b+5=25(a+b+5)$
$\Rightarrow 15a=3b+24$
Nếu $a=1$ thì $b=-3$ ( loại )
Nếu $a=2$ thì $b=2$
Nếu $a=3$ thì $b=7$
Vậy $(a,b,c)=(2,2,5)$ hoặc $(a,b,c)=(3,7,5)$