If n is an integer and f(n) = f(n 1) n, what is the value of

Question

If n is an integer and f(n) = f(n – 1) – n, what is the value of f (4)?

(1) f(3) = 14

(2) f(6) = -1

pike · Answer

f(4) = f(3) - 4

1) Sufficient, slot straights into above equation.

2) 
 f(6) = -1 = f(5) - 6
 f(5) = 5 = f(4) - 5
 f(4) = 10
 
 Sufficient

D.

Bunuel · Answer

If n is an integer and f(n) = f(n – 1) – n, what is the value of f (4)?

(1) f(3) = 14. Since f(n)=f(n-1)-n then: f(4)=f(3)-4 --> f(4)=14-4=10. Sufficient.

(2) f(6) = -1. The same way as above f(6)=f(5)-6 --> -1=f(5)-6 --> f(5)=5 --> f(5)=f(4)-5 --> 5=f(4)-5 --> f(4)=10. Sufficient.

Answer: D.

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dcastan2 · Answer

I don't understand what you mean slot straights into above equation. When I simplify the f(3) I get -1 and for f(6) I also get -1. Is that why both are sufficient? I don't understand why Statement 1 the f(3)=14. What I supposed to do something else?

Marcab · Answer

Hii.
See the question gives us the relation:  f(n)=f(n-1)-n  and the question asks the value of  f(4) .

As per the above relation, if we expand  f(4) , we get:  f(4)=f(3)-4 ---------

Now coming to the statements:
Statement 1 gives us direct  f(3) . We just have to put in equation  .
Statement 2 gives us  f(6)=-1 .
 f(6)  can be expanded as  f(5) -6. Moreover  f(5)  can be expanded as  f(4)-5 . Put this value of  f(5)  in the former one. It will become  f(6)=f(4)-9 , which will gives the value of  f(4)  as 10.

Both are sufficient.
Hope that helps.

dcastan2 · Answer

Yes, thank you! But we don't use the =14 anywhere? What's the purpose of it then?

Marcab · Answer

In statement 1, put f(3)=14. You will get the answer as 10.
In DS, if you know that with the given information a particular question can be solved then rather than trying to find the exact answer, move to the next statement.
In the explanation, I did the same.

Hope that helps.

Edvento · Answer

The equation says f(n) = f(n-1) - n
(1) states f(3)=14 and we need to find f(4)
so as per the equation, f(4) = f(4-1) -4 
==> f(4)=f(3)-4
==> f(4)=14-4=10
So (1) is sufficient...Hope explanation for (2) is already clear. And this expalins the use of f(3)=14 as well.
So the answer is D

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If n is an integer and f(n) = f(n 1) n, what is the value of

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If n is an integer and f(n) = f(n 1) n, what is the value of

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