Question

If all the books on a shelf with fewer than 45 books were put into piles of five books each, no books would remain. If the same book were put into piles of seven books each, two books would remain. What is the greatest number of books that could be on the shelf?

Answer 1

Hence the greatest number of books that could be on the shelf is 30.

Explanation:

Step 1:-

Here the given number of books is m,

m < 45.

If they are arranged into piles of five books, each no books would remain.

m is divisible by 5.

The last digit of m would be 5 or 0.

The same number of books, when arranged with piles of 7 books each, two books remain.

m-2 is divisible by 7.

Step 2:-

The last digit of m is 5 or 0.

m-2 will have the last digit 3 or 8.

Multiples of 7 are 7,14,21,28,35, 42;

among which only number 28 has the last digit 8 or 3.

m - 2 = 28

m = 30

The greatest number of books that could be on the shelf is 30.