## adnanchowdhury 3 years ago How many different numbers can be formed by taking one, two, three and four digits from the digits 1, 2, 7 and 8, if repetition are not allowed?

1. bahrom7893

1 digit - 4 ways (any one can be chosen)

2. bahrom7893

2 - digits - 4*3 ways (you picked the first one, so now you have 3 to choose from)

3. bahrom7893

Waaait, isn't it 4C1

I don't understand. Can you list the permutations? I can't seem to understand what the question is saying.

Answer is supposed to be 64.

6. bahrom7893

For which one?

7. bahrom7893

wait im wrong.. hold up. You have to use that formula, like the number of ways of choosing some object from a set of objects with no repetitions

8. bahrom7893

ehhh... im lost.. crap i suck at probability

9. bahrom7893

Let me call amistre

Can you explain what the question is asking? What does it mean when it says: 'take one, two, three and four digits from the digits 1, 2, 7 and 8'?

11. bahrom7893

im thinking: Pick one digit from 1,2,7 and 8. Like I can pick 1, or 2, or 7, or 8, so the answer's 4 ways

12. bahrom7893

yay amistre's here

So would one possibility be 1278?

One digit possibilities: 1, 2, 7 and 8. So there are 4 permutations for one digits...?

I understand now!

4 possibilities for 1 digits. 4x3 = 12 possibilities for 2 digits. And so on..

17. bahrom7893

yes that's what i was trying to say adnan

Thanks! My problem was in understanding what the question was asking, but after you explained it, i understood.

19. amistre64

1, 2, 7 and 8 1 digit numbers; there are only 4 numbers so: 4 2 digit number: 12 17 18; 4 times = 12; 4P1 * 3P1 = 12 3digit number: 127 128 172 178 182 187; 6*4 = 24; 4*3*2 = 24 4 digit numbers: 1278 1287 1728 1782 1827 1872: 6*4 = 24; 4*3*2*1 = 24 4+12+24+24 = 4+12(3) = 40 i beleive

20. amistre64

lol, almost had that right

21. amistre64

60+4 if i learn to count :)

yes lol

Any formula i could have used to save me time?

24. amistre64

not really; since it is split up into 4 groups to count individually you still have a bit of work

25. amistre64

knowing the cPr stuff helps tho

26. amistre64

nPr ... cant type

I understood the formula before. But now, I just find it confusing.

28. amistre64

yeah, its just a "structured" way to count and if you dont use it you tend to go back to the basic method of 1,2,3,4,5s