Помогите решить:(3/14+5/21у):3/7=З 1/4

$(\frac{3}{14}+\frac{5}{21}y):\frac{3}{7}=3 \frac{1}{4}$

$3\frac{1}{4}=\frac{3*4+1}{4}=\frac{13}{4}$

$\frac{3}{14}+\frac{5}{21}y=\frac{13*3}{4*7}$
$\frac{3}{14}+\frac{5}{21}y=\frac{39}{28}$
домножим на 7*3*2*2, получим
$3*6+4*5y=39*3$
$18+20y=117$
$20y=118-18$
$20y=99$
$y=\frac{99}{20}$
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$(\frac{3}{14}+\frac{5}{21}y):\frac{3}{7}=3 \frac{1}{4}$
$(\frac{3}{14}+\frac{5}{21}y)*\frac{7}{3}=\frac{13}{4}$
$\frac{1}{2}+\frac{5}{9}y=\frac{13}{4}$
$\frac{5}{9}y=\frac{13}{4}-\frac{1}{2}$
$\frac{5}{9}y=\frac{11}{4}$
$y=\frac{11*9}{4*5}=\frac{99}{20}$