(21/22-5/11)+(22/39-3/13)

21/22  -  5/11 =(21 × 1  -  5 × 2)/22 = 21/22  -  10/22 =  11/22 = 1/2
22/39  -  3/13 =(22 × 1  -  3 × 3)/39 = 22/39  -  9/39 =  13/39 = 1/3
1/2  +  1/3 =1 × 3  +  1 × 2)/6 = 3/6 +  2/6 =  5/6

(21/22-5/11)+(22/39-3/13)=(21/22-10/22)+(22/39-9/39)=11/22+13/39=1/2+1/3=(3+2)/6=5/6