A 180 liter mixture of syrup and water contains 20% of syrup. What quantity of syrup must be added with that mixture to get 25% syrup?

Total mixture = 180 liter
Syrup contains = 20%

Quantity of syrup in solution = 180 × 20% =180 × (1/5) = 36liter
Quantity of water in mixture = (180 ─ 36) liter = 144 liter
Now in mixture contain 25% of syrup
∴ Water = (100% ─ 25%) = 75%

According to the question,
⇒ 75x = 144
⇒ x = 144/75

∴ Total mixture = 100x liter = 100 × (144/75) = 192 liter
∴ Syrup should be added in mixture = (192 ─ 180) = 12 liter