Let f(x) denotes the sum of the digits of the positive integer x. For example, f(8) =8, f(123) = 1+2+3=6. For how many two-digit values of x is f(f(x))=3?
A) 3
B) 4
C) 6
D) 9
E) 10
The OA is E.
Can any expert help me with this PS question please? I don't have it clear. Thanks.
Let f(x) denotes the sum of...
This topic has expert replies
-
- Moderator
- Posts: 2207
- Joined: Sun Oct 15, 2017 1:50 pm
- Followed by:6 members
- EconomistGMATTutor
- GMAT Instructor
- Posts: 555
- Joined: Wed Oct 04, 2017 4:18 pm
- Thanked: 180 times
- Followed by:12 members
Hello LUANDATO.
It is a really interesting question.
The idea is that we must find the numbers of two digits such that the sum of their digits is:
- 3, that is to say, f(x)=3, so that f(f(x))=f(3)=3.
- 12, that is to say, f(x)=12, so that f(f(x))=f(12)=3.
The numbers that satisfy this property are: 12, 21, 30, 39, 48, 57, 66, 75, 84 and 93.
f(f(12))=f(3)=3
f(f(21))=f(3)=3
f(f(30))=f(3)=3
f(f(39))=f(12)=3
f(f(48))=f(12)=3
f(f(57))=f(12)=3
f(f(66))=f(12)=3
f(f(75))=f(12)=3
f(f(84))=f(12)=3
f(f(93))=f(12)=3
So, the answer is E.
Note that f(f(3))=3, but 3 is one-digit number, that is why it is not on the list above.
I hope this can help you to understand how to solve this kind of questions.
It is a really interesting question.
The idea is that we must find the numbers of two digits such that the sum of their digits is:
- 3, that is to say, f(x)=3, so that f(f(x))=f(3)=3.
- 12, that is to say, f(x)=12, so that f(f(x))=f(12)=3.
The numbers that satisfy this property are: 12, 21, 30, 39, 48, 57, 66, 75, 84 and 93.
f(f(12))=f(3)=3
f(f(21))=f(3)=3
f(f(30))=f(3)=3
f(f(39))=f(12)=3
f(f(48))=f(12)=3
f(f(57))=f(12)=3
f(f(66))=f(12)=3
f(f(75))=f(12)=3
f(f(84))=f(12)=3
f(f(93))=f(12)=3
So, the answer is E.
Note that f(f(3))=3, but 3 is one-digit number, that is why it is not on the list above.
I hope this can help you to understand how to solve this kind of questions.
GMAT Prep From The Economist
We offer 70+ point score improvement money back guarantee.
Our average student improves 98 points.
We offer 70+ point score improvement money back guarantee.
Our average student improves 98 points.