12/13-15/26 = 24/26 - 15/26 = 9/26
9/28-11/35 = 45/140 - 44/140 = 1/140
38/40-19/28 = 28/40 - 19/28 = 7/10 - 19/28= 28/40 - 19/28= 14/20 - 19/28= (14 14)/(20 14) - (19 10)/(28 10)= 196/280 - 190/280= 6/280= 3/140
12/13-15/26 = 24/26 - 15/26 = 9/26
9/28-11/35 = 45/140 - 44/140 = 1/140
38/40-19/28 = 28/40 - 19/28 = 7/10 - 19/28
= 28/40 - 19/28
= 14/20 - 19/28
= (14 14)/(20 14) - (19 10)/(28 10)
= 196/280 - 190/280
= 6/280
= 3/140