Question

how many number can be formed using the digits 1,7,4, and 5 exactly once ​

Answer 1

if you will take 1 in the extreme right place then you can form 6 numbers

this law is same for every number

so there are four numbers

1,4,5,7

so you can form

6*4 numbers

that is

24 numbers

some of them are

1457

1475

1547

1574

1754

1745

do same as I have done taking one constant in the first place

:)

Answer 2

1745

1754

1457

1475

1547

1574

7451

7415

7145

7154

7514

7541

4517

4571

4715

4751

4175

4157

5174

5147

5417

5471

5714

5741

hope these are enough...