Solution: Because according to the meaning of the title, the total number of books is 5x 1+4=9.

And if five people each have two books, then the required books are 5x2= 10.

But 9 < 10, so if everyone gets two copies, that's not enough.

Properties of addition operation

From additive commutative law's associative law, it can be concluded that when several addends are added, the positions of addends can be exchanged at will; Or add a few addends first and then add them with other addends, and the sum remains the same.

Subtraction operation property

Subtracting the sum of two numbers from a number is equivalent to subtracting each addend in the sum from this number in turn.

A number minus the difference between two numbers is equal to this number minus the minuend in the difference, and then the subtraction.

Multiplication operation property

Multiply the product of several numbers by a number so that any factor in the product can be multiplied by this number and then multiplied by other numbers. For example: (25×3 × 9)×4=25×4×3×9=2700.

Multiply a number by the difference between two numbers, and you can multiply the minuend and the minuend by this number respectively, and then subtract the product. For example: (137-125) × 8 =137× 8-125× 8 = 96.

Refer to the above content: Baidu Encyclopedia-Four Arithmetic