(1) With four digital cards of 1.2.3.5, how many different two-digit numbers can be displayed? Let it out first, then write it out.

* * * can output 12 different two-digit numbers. They are 12, 13, 15, 2 1, 23, 25, 3 1, 32, 35, 5 1, 52, 53.

The prime numbers are: 2,3,5,13,23,3153.

The composite numbers are: 12,15,21,25,32,35,51,52.

Odd numbers are: 1, 3,5, 13,15,21,23,25,31,35,51,53.

Even numbers are: 212,32,52.

There are common factors 2: 2 and12; 2 and 32; 2 and 52. 12 and 32; 12 and 52; 32 and 52.

There is a common factor of 3; 3 and15,3 and 2 1, 3 and 51; 15 and 21; 15 and 51; 2 1 and 5 1

The common multiples of 2 and 3 are: 12,

Common multiples of 3 and 5 are: 15,

I hope I can help you. @ Math Tutoring Group # I wish you progress in your study. Please ask questions if you don't understand, and adopt them in time if you understand! (*^__^*)