Question

How many 3 digit numbers can be formed using the digits 1, 3, 5, 7, 9, where we are

allowed to repeat the digits?

(a) 125 (b) 25 (c) 5 (d) None of these​

Answer 1

Answer:

since each desired number is divisible by 5 , so, we must have 5 at the unit place,so, there is 1 way of doing

The tens place can now be filled by any of the remaining digits (2,3,6,7,9,) so, there are 5 ways of filling the tens place

the hundreds place can now be filled by any of the remaining 4 digits , so, there are 4 ways of filling it

therefore , required number of numbers (1×5×4) =20