Помогите с объяснением 11 баллов и столбиком

Решите задачу:

$(7 \frac{1}{5} -2 \frac{13}{100} ):1,3 = 3,9$

$1) 7 \frac{1}{5}-2 \frac{13}{100} = \frac{7*5+1}{5}- \frac{2*100+13}{100} = \frac{36}{5} -\frac{213}{100} = \frac{20*36-213}{100} = \frac{720-213}{100} = \frac{507}{100}= 5,07$

$2) 5,07 : 1,3 = 3,9$

$(2 \frac{5}{21} - 2,2): \frac{2}{105} = 2$

$1) 2 \frac{5}{21} -2,2= \frac{2*21+5}{21} - \frac{22}{10}= \frac{47}{21} -\frac{22}{10}= \frac{10*47-21*22}{210} = \frac{470-462}{210} = \frac{8}{210} = \frac{4}{105}$

$2) \frac{4}{105} : \frac{2}{105} = \frac{4}{105} *\frac{105}{2} = 2$

$(1 \frac{3}{4} +1,7) : \frac{1}{20} =69$

$1) 1 \frac{3}{4} +1,7 = \frac{1*4+3}{4} + \frac{17}{10}= \frac{7}{4} + \frac{17}{10} = \frac{10*7+4*17}{40} = \frac{70+68}{40} = \frac{138}{40} = 3,45$

$2) 3,45: \frac{1}{20} = 3,45:0,05 = 69$