5/12 а +7; 2) 4/9 b +2/3 при b =6; 3)2/5+1.1/4a при а =2; 4) 2.3/10b+1/2 при b =5

(5/12)а + 7 при а=4 4/9
то:
а=4 4/9 = (4*9+4)/9=40/9
(5/12)(40/9) + 7=(175/36)+7=427/36 = 11 31/36

если так:
5/(12а) + 7 при а=4 4/9
то:
а=4 4/9 = (4*9+4)/9=40/9
5/(12*40/9) + 7=(3/32)+7=227/32 = 7 3/32

второй аналогично

4/9b+2/3≈4/9*6+2/3≈8/3+2/3≈10/3≈3 1/3
2/5+1 1/4a≈2/5+1 1/4*2≈2/5+5/2≈20/10≈2
2 3/10b+1/2≈2 3/10*5+1/2≈23/2+1/2≈24/2≈12