Build a library with a book, there are

A x 2/5 =320

therefore

A=320？ ÷2/5

= 1600

If you are familiar with multiplication and its inverse operation (division), you can pass 320? ÷2/5 is used for calculation.

Extended data:

Multiplication operation satisfies: commutative law, associative law, distributive law and elimination method.

1, multiply method of substitution: ab=ba. Note that letters are multiplied by letters, and the multiplication sign can be written without writing.

2. Multiplicative associative law:? (ab)c=a(bc).

3. Multiplication distribution law: (a+b)c=ac+bc.