Bheja fry question

 This question I heard it recently on web radio.. 

Question

A waste is in the lake.

It doubles its size each day.

On the 20th day, it covers the whole lake.

On what day, waste covers half of the whole lake  ???

Answer

By reading the question, we tend to answer the question as 10th day..

Because I answered it like that   !!!

But answer is not correct.

Read the second sentence carefully. 

It reads "  it doubles each day ".

This means,

If the waste covers full lake on 20th day, 

IT HAS TO COVER HALF OF THE LAKE ON 19TH DAY.

We must not worry what has happened between first and 19th day. 

So answer is on 19th day

Not 10th day !!!!