Question 77180
Dear Student,

You have to work cool to arrive at the answer:
Let us add x ml of grape juice to the old juice
 
Old Juice...800   ml....15%grape=120....ml grape juice
new Juice...800+x ml....20%grape=120+x..ml grape juice

now we get (120+x)/(800+x)=20/100  (ie 20%=20/100)
solving the first degree eqn

(120+x)/(800+x)=20/100  
100(120+x)=20(800+x)
expanding 12000+100x=16000+20x
100x-20x=16000-12000
80x=4000
x=4000/80=50 ml
so you add 50 ml of fresh grape juice to make it 20% conventrate.

Cross verification:

new juice...grape=120+50=170 ml
nrw juice  total =800+50 =850 ml
so % grape =170/850=20%
okay
good day !!