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A tank can hold 50 litres of water. At present, it is only 30 % filled. How many litres of water shall we put in the tank so that its 50 % filled?

Answer
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Hint: To solve this question, we will find what is 1 % of the total volume of the tank. Then, we will find the litres of water when the tank is 30 % filled and litres of water when the tank is 50 % filled. Then we will find the difference between these values and the result will be the litres of water we shall put in the tank so that it increases from 30 % filled to 50 % filled.

Complete step-by-step answer:
It is given that the tank can hold 50 litres of water and given that 30 % of the tank is filled.
Thus, 1 % of the 50 litres is given as $ \dfrac{1}{100}\times 50 $ litres = 0.5 litres.
So, 30 % of the 50 litres capacity tank is given as the product of 1 % of 50 litres capacity tank and 30%.
Therefore, 30 % of 50 litre capacity tank = 0.5 $ \times $ 30 = 15 litres
Thus, out of 50 litres capacity, the tank has 15 litres of water.
Now, 50 % of the 50 litres capacity tank is given as the product of 1 % of 50 litres capacity tank and 50%.
Therefore, 50 % of 50 litre capacity tank = 0.5 $ \times $ 50 = 25 litres
Thus, we want the tank to be filled up to 25 litres.
So, the extra water we need to fill is equal to the difference of 50 % and 30 % of the tank capacity.
Thus, extra water = 25 – 15 = 10 litres

Note: Students can directly solve this question by finding the difference between 50 and 30, i.e. 20 % and find the 20% of the total tank capacity. Moreover, they are advised to be careful with the units and write it wherever needed as answers without units are considered incomplete.