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I have some sat questions to confirm. when i keep solving a few of the math problems i keep getting one answer, but the answer page says another. Please help!!!!! Include explanations!

1. What is the units digit of 3^155?
a) 1
b) 3
c) 5
d) 7
e) 9

By mike on Saturday, February 15, 2003 - 09:51 pm: Edit

the answer is 1
this is how you do it..
figure out the units digit in 3^1, 3^2, 3^3 and 3^4 is.

the pattern is 3,9,7,1(if you figure out what 3^5 is, the units digit is 3 which starts the pattern all over again.)

then, you divide 155 by 4 and get 38.75. So multiply 38 by 4(the amt of #'s in the pattern) and you get 152. So that means 155 would be the third number in the pattern, thus, 1.

By bystander on Saturday, February 15, 2003 - 10:49 pm: Edit

um... isn't the answer 7 (d).

mike, your reasoning is sound, except for the last part i think. the third number in the pattern is 7 because 3^3 is 27.

or maybe i am just an idiot.

By mike on Saturday, February 15, 2003 - 11:05 pm: Edit

lol yeah ma bad.. i was rushing and wrote my 7 like a 1.

u are correct

By befuddled on Sunday, February 16, 2003 - 10:05 am: Edit

are you sure, because the answer booklet says c) 5, and i think the answer key was wrong but i am not sure.

By Cornellian 2007 on Sunday, February 16, 2003 - 10:16 am: Edit

It could never be 5, since 5 is not in the pattern. The answer is in fact 7.