21/13:x=7/26; 21/13 * 1/x=7/26 ; 21/13x = 7/26; 13x*7=21*26;
x=21*26 / 13*7; x= 3*2; x=6
y*20/11=5/33 ; y*20*33 = 5*11; y=5*11 / 20*33 ; y=1/4*3; y=1/12
21/13:x=7/26; 21/13 * 1/x=7/26 ; 21/13x = 7/26; 13x*7=21*26;
x=21*26 / 13*7; x= 3*2; x=6
y*20/11=5/33 ; y*20*33 = 5*11; y=5*11 / 20*33 ; y=1/4*3; y=1/12