Question

Stranded on deserted island, harry Potter is left with only a 40 litre container of milk. To conserve his milk he decides that on the first day he will drink one litre of milk and then refill the container backup with water. On the 2nd day he will drink 2 litre and refill the container. On the 3rd day he will drink 3 litre and so on . . ... By the time all the milk is gone, how much water has he drunk?
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Answer 1

Given :

Harry has 40 litres of milk in container.

He drinks 1 litre on first day from container, 2 litre on second day, 3 litre on third day and goes on.

To find :

Quantity of water drunk by harry when all milk is gone.

Solution :

Harry drink one litre of milk on first day , then he add up one litre of water in it and then on second day he drink 2 litres of that mixture,he drinks 3 litre of mixture on third day and it goes on.

Total there is 40 litres of milk in starting .

So, he will drink 40 litre of mixture on 40th day so, all the milk was empty on 40th day.

We have to calculate the amount of water he added in these 40 days because he drank all the water thay he added in 40 days.

So, on 1st day, he added 1 litre of water after drinking 1 litre of milk, similarly 2 litre on second day...

On 39th day he will add 39 litres of water in container,and on 40th day all the mixture was empty and he will not add anything.

So

Formula of adding n natural numbers :

$\\ \bf \: = \frac{n \times (n + 1)}{2}$

(equation 1)

In this case value of n = 39

so putting value of n = 39, in equation 1, we get

Total quantity of water added :

$\bf = \frac{39 \times 40}{2} = 39 \times 20 = 780$

So,

Harry added total 780 litres of water till all the milk is empty.

Answer 2

Answer: 780

Step-by-step explanation: assume milk to be in vessels in packets, and assume man is topping up with a 1 litre water packets.