Answer:
See below
Explanation:
1) Let us tabulate and see.
Sugar content Water Total weight
I syrup 0.3(35) = 10.5 35-10.5 = 24.5 35
II syrup 0 x x
total 10.5 24.5+x 35+x
concentration in new syrup=

7kg of water should be added.
2) John can do 3/6 apps in one week while Ben can do 2/6 apps in one week.
Together they can do 5/6 apps per week.
Hence to do 10 apps they require 12 weeks.
3) 120% of a = 80% of b
i.e. 1.2a = 0.8b
Or 3a=2b
Or a:b = 2:3
By commendo and dividendo we get
a:a+b= 2:5
Or a+b = 5a/2 ... i
Next is
a+b:b =5:3
Or a+b= 5b/3