Question

Find the sum of unit digits of all different numbers formed from digits 4, 6, 7 and 9.

Answer 1

Answer:

Four digit numbers =4⋅3⋅2⋅1=24 ways we can form a four digit number. Since it's a 4 digit number, each digit will appear 6=24/4 times in each of units, tens, hundreds, and thousands place. Therefore, the sum of digits in the units place is 6(1+2+5+6)=84.

Why they are adding 1+2+5+6?

Similarly sum of digits at tens,hundreds,thousands place is 84

Required sum of all numbers  =84+84⋅10+84⋅100+84⋅1000=93324

Step-by-step explanation: