Hi..

Firstly on the question on greater than 10 paperback and less than 11 hardback..

So let us see the maximum and minimum.

1) max books..

Let all be of paperback and money be equal to 300, so 300/8=37.5

2) Min books..

Let the max be of hardback and price be closer to 240

So 11*8=88 and remaining will be hardback, so (240-88)/20=152/20=~7.5 =8

Thus 11+8=19

So 19 to 37..

In this context the explanation is wrong as there second portion is almost similar to first and that is why they get 19,20,21 in both cases.

Now onto the second question..

1) atleast one of 40 and one of 60..

Total 50 with average more than 50..SO more of 60..

Let's see min and max that for 40 ones to be added to get it to below 50..

Min... When there is least difference between the two so 26 of 60 and 24 of 40..

If we add two of 40, both will be equal, 26 each, and the average will be midway at 50, but we want it below 50, so add one more of 40.. Thus, min is 2+1=3..

Max... When there is max difference so 1 of 40 and 49 of 60.. can be anything

So 1 and 2 are wrong

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Some useful Theory.

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