BREAKING: Target Test Prep releases Brand New 2026 On Demand GMAT prep course

Redeem

Units Digit Problem

This topic has expert replies
Post new topic Post Reply
Previous Topic Next Topic
Source: — Problem Solving |
User avatar
Legendary Member
Posts: 1101
Joined: Fri Jan 28, 2011 7:26 am
Thanked: 47 times
Followed by:13 members
GMAT Score:640

by HSPA » Tue Mar 15, 2011 11:54 pm
powers of 6: 6,36,216... ending 6 always
powers of 7: 7,49,343,XXX1.... ending in 7,9,3,1,7... repeat
powers of 9: 9,81,729... ending in 9,1,9,1..repeat

Now using all unit digit endings (9^3+7^4) = 9+1 = 10
Now 6-0 = 6
Top
User avatar
Master | Next Rank: 500 Posts
Posts: 261
Joined: Wed Mar 31, 2010 8:37 pm
Location: Varanasi
Thanked: 11 times
Followed by:3 members

by ankur.agrawal » Wed Mar 16, 2011 12:41 am
6983manish wrote:What is the units digit of 6^15 - 7^4 - 9^3?
A) 8
B) 7
C) 6
D) 5
E) 4
I have seen multiples between the two numbers .For the first time I see a subtraction.

ok 6^15 ---> unit digit is 6

7^4 --> unit digit is 1

9^3 ---> unit digit is 9 . Hmm ?

( 6^15 - 7^4) - 9^3 = Using Unit digit endings

( 6-1) - 9 = 5-9 -----> take a carry so 15-9=6 ?

Is this the correct way to do it . Pls comment.
Top
Post new topic Post Reply
• Page 1 of 1