六年级简便计算题

3.1+2.4+1.9+3.6
=(3.1+1.9)+(2.4+3.6)
=5+6
=11

4.2*99+4.2
=4.2*(99+1)
=4.2*100
=420

42/38
=2*21/2*19
=21/19

24*（3/8+5/6）
=24*3/8+24*5/6
=9+20
=29

9.7*1.25*0.8
=9.7*(1.25*0.8)
=9.7*1
=9.7

(7/9)/(5/11)+(5/11)*(2/9)
这个是不是(7/9)*(5/11)+(5/11)*(2/9)？
如果是则
(7/9)*(5/11)+(5/11)*(2/9)
=(5/11)*(7/9+2/9)
=(5/11)*1
=5/11

(1)11
(2)420
(3)21/19
(4)29
(5)9.7
(6)299/165