When a book of 100 pages is opened, the sum of the facing page numbers can be divided by 5, the quotient is a product of 3 and 7, what are the facing page numbers?

the answer is 52 and

what is the calculation process

Sum means the result of addition.
Quotient means the result of division.
Product means the result of multiplication.

The product of 3 and 7 is 21, because:
3 × 7 = 21

Therefore, the quotient is 21.

Let the sum of the facing page numbers = x:
Therefore, x/5 = 21
Rearranging the equation to find x (the sum of the facing page numbers) gives us:
x = 21 × 5
= 105
So the sum of the facing page numbers is 105.

The only two adjacent numbers which add up to 105 are 52 and 53, so the facing page numbers must be 52 and 53.