In a mixture of 45 litres, the ratio of sugar solution to salt solution is 1 :…

First, calculate the initial amounts of sugar and salt in the 45-litre mixture. With a ratio of 1:2, sugar is 1/3 of the total (15 litres) and salt is 2/3 (30 litres). Since only sugar is added, the amount of salt remains constant at 30 litres. To achieve a new ratio of 2:1 (sugar to salt), the sugar amount must be double the salt amount, which is 60 litres. Therefore, the amount of sugar to add is the difference between the required 60 litres and the initial 15 litres, which equals 45 litres.