how many number of telefone with 6 numbers( 2 number 2, 2 number 5, 2 number 7)?

Question

a.50 c.70 c 90 d 110

Answer 1

6!/2!2!2! = 90

You are partitioning the 6 positions in the number into 3 groups of 2.
A group to hold the 2's a group to hold the 5's and a group to hold the 7's. This number is what is known as a multinomial coefficient.

Answer 2

6 number in 6! ways but because there are repetitions, discount this by reptitions.

So, total number of combinations possible - 6!/2!2!2! = 90

Answer 3

I am just guessing.... since there are 3 different numbers I expect that the solutions must be divisible by 3 thus I choose 90