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8/3 - 5/3 : 20/21
8/3 - 5/3 * 21/20
8/3 -  5*7*3 / 3*4*5 = 8/3 - 7/4
8*4 / 3*4 - 7*3 / 3*4 = 32/12 - 21/12 = 11/12

(2 + 2/3) : ( 4/5 - 2/3)
(2*3 / 2*3 + 2*2 / 3*2) : ( 4*3 / 5*3 - 2*5 / 3*5 ) 
(6/6 + 4/6) : (12 /15 - 10/15)
10/6 : 2/15
10/6 * 15/2
5*2*5*3 / 3*2*2 = 25/2 = 12.5