Solution:
As, it is given that school's lemonade is $10\%$ lemon juice by volume, and the rest is sugar water.
School's lemonade which is Sugar water= (100-10)=90 % volume
Let total volume of Lemonade (Lemon Juice + Sugar Water) = x liter
Volume percent of Lemon Juice = 
=
%
Let , number of liters of lemon juice must be added to 20 liters of sugar water to make the school's lemonade= x liter
To find , number of liters of lemon juice must be added to 20 liters of sugar water to make the school's lemonade, we will use the following formula:
→Volume percent of Lemon Juice = 
= 
= 2.23 Liter(Approx)