In sequence Q, the first number is , and each subsequent number in the sequence is determined by doubling the previous number and then adding 2. How many times does the units' digit 8 appear in the first 15 numbers of the sequence?
A)7
B)8
C)10
D)12
E)13
E
Please explain your answer... need help with this one.. thanks!
Sequence question
This topic has expert replies
- cans
- Legendary Member
- Posts: 1309
- Joined: Mon Apr 04, 2011 5:34 am
- Location: India
- Thanked: 310 times
- Followed by:123 members
- GMAT Score:750
f(n)=2f(n-1)+2=2(f(n-1)+1)
for unit number 8, f(n-1)+1 should end either in 9 or in 4.
thus f(n-1) should end in 8 or 3.
f(1)=1,f(2)=3,f(3)=8
now as f(3) ends in 8, f(4) will end in 8 and thus f(5) will also end in 8 and so on....
thus till f(15), 13
IMO E
for unit number 8, f(n-1)+1 should end either in 9 or in 4.
thus f(n-1) should end in 8 or 3.
f(1)=1,f(2)=3,f(3)=8
now as f(3) ends in 8, f(4) will end in 8 and thus f(5) will also end in 8 and so on....
thus till f(15), 13
IMO E
If my post helped you- let me know by pushing the thanks button
Contact me about long distance tutoring!
[email protected]
Cans!!
Contact me about long distance tutoring!
[email protected]
Cans!!
cans wrote:f(n)=2f(n-1)+2=2(f(n-1)+1)
for unit number 8, f(n-1)+1 should end either in 9 or in 4.
thus f(n-1) should end in 8 or 3.
f(1)=1,f(2)=3,f(3)=8
now as f(3) ends in 8, f(4) will end in 8 and thus f(5) will also end in 8 and so on....
thus till f(15), 13
IMO E
I am sorry, I missed out something in my question..
"In sequence Q, the first number is 1/2 "
so .... if this information is given.. is plugging in good ? .. how to start of with this question ?
-
- Legendary Member
- Posts: 966
- Joined: Sat Jan 02, 2010 8:06 am
- Thanked: 230 times
- Followed by:21 members
Just list down the numbers;
1st : 1/2
2nd : 3
3rd : 8
4th : 18 (Now you know 8*2 =16, so adding 2 will always end in 8)
So from the 3rd to 15th term all will end in 8.
Thats 13 terms
1st : 1/2
2nd : 3
3rd : 8
4th : 18 (Now you know 8*2 =16, so adding 2 will always end in 8)
So from the 3rd to 15th term all will end in 8.
Thats 13 terms