20/(x/9)=4

х/9=20/4

х=5*9

х=45

проверка

20/(45/9)=4

20/5=4

4=4

Verified answer

20/(x/9)=4

20*9/х=4

180/х=4

х=180*1/4

х=45

20/45:9=4

20/5=4

4=4