(4/5 - 11/15) х 5/11 =((4*3-11)/15)*5/11=
=1/15*5/11=1/33
2) 7/88 × (8 / 21+ 8/7) =7/88*(8*2+8*6)/42=
=7/88*(16+48)/42=
=7/88*64/42=32/264=17/132
3) 3/7×(1/4+7/12)=3/7*(1*3+7)/12=3/7*10/12=10/28=5/14
