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(1) [1,2] books checked out by 80 patrons => [1,2]*80=[80,160] books (minimum and maximum)
Books left = 1600 - [80,160] = [ 1440,1520 ] books (minimum and maximum) left to patrons that checked out between 3 and 10 books each.
Minimum #patrons =80 + min([1440,1520]/[3,10]) = 80 + 144 < 300
Since "Minimum #patrons < 300", we need calculate the maximum value of #patrons to evaluate if #patrons < 300
Maximum #patrons = 80 + max([1440,1520]/[3,10]) = 80 + 1520/3 > 300
Insufficient information.

(2) [3,4] books checked out by 150 patrons => [3,4]*150 = [ 450,600 ] books (minimum and maximum)
Books left = 1600 - [450,600] = [1000,1150] books (minimum and maximum) left to patrons that checked out between 1 and 2 books each, and patrons that checked out between 5 and 10 books each.
Minimum #patrons =150 + min([1000,1150]/[5,10]) = 150 + 100 < 300
Since "Minimum #patrons < 300", we need calculate the maximum value of #patrons to evaluate if #patrons < 300
Maximum #patrons = 150 + max([1000,1150]/[1,2]) = 150 + 1150/1 > 300
Insufficient information.

(1 and 2)
Books left = 1600 - [80,160] - [450,600] = [840,1070] books (minimum and maximum) left to patrons that checked out between 5 and 10 books each.
Minimum #patrons =80 + 150 + min([840,1070]/[5,10]) = 230 + 84 > 300
Sufficient information => (C)
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In such questions, we can define min & max ranges for clarity. Below is the solution considering the ranges. Note: the logic we need to test is that if given people took max books they can for eg in statement 1 it was 2 people, and then the remaining also took the max they can (cap is 10 books) that will give us the minimum number of people taking maximum number of books.
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mmdfl
At a certain library, 1600 books were checked out by patrons yesterday. If each patron checked out at least 1 book and at most 10 books yesterday, was the number of patrons who checked out books yesterday greater than 300?

(1) 80 patrons checked out 1 or 2 books each.
(2) 150 patrons checked out 3 or 4 books each.

Application of "use the information you have".

Let's see if 1600 books could be checked out by 300 patrons. If not, then the number of patrons would need to be more than 300.

I will review statement 2 first here because it gives bigger numbers and hence makes it more likely that my constraints are satisfied.

(2) 150 patrons checked out 3 or 4 books each.


It means 450 to 600 books are accounted for by 150 people. Now we have 150 people left who need to account for leftover books (at most 1150 are left)
Each of the 150 people could have checked out 10 books and hence they can account for up to 1500 books. So number of people can certainly be less than 300.
Not sufficient alone.

If (2) is not sufficient alone, (1) alone cannot be since 220 people can account for 2200 books, much more than 1600.

Using both together:
From stmnt 1, 80 patrons can account for at most 160 books. (Each checks out 2 books)
From stmnt 2, 150 patrons can account for at most 600 books. (Each checks out 4 books)
Now I have 300 - 150 - 80 = 70 patrons left. They can account for at most 700 books. (Each checks out 10 books)
We can account for only 160+600 + 700 = 1460 books. But we need to account for 1600 books.
Hence there must have been more than 300 patrons.

Answer (C)


Another question in which you should start with what you have been given:
https://youtu.be/Nlb6hg-njvQ
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