GMAT prep - Questions - Test 1

[tutonaranjo]

Please help with the following questions.

1) In the infinite sequence a1, a2, a3,....,an...; each term after the first is equal to twice the previous term. If a5 - a2 = 12, what is the value of a1?
a) 4
b) 24/7
c) 2
d) 12/7
e) 6/7

Answer is E

[gabriel]

.. Hi there ..
as per the rules of the forum ur supposed to post only one question per post/thread .. it helps others keep a track of the questions being discussed .. I am breaking up ur post and making separate threads for each one .. Thnx ..

[gabriel]

Please help with the following questions.

1) In the infinite sequence a1, a2, a3,....,an...; each term after the first is equal to twice the previous term. If a5 - a2 = 12, what is the value of a1?
a) 4
b) 24/7
c) 2
d) 12/7
e) 6/7

Answer is E

Ok, so here each term is twice the previous one .. so what does that mean ..

Well it means a2= 2*a1, a3=2*a2,a4=2*a3,a5=2*a4 ..

Now, we have a5-a2=12 but a5=2*a4.. so that would mean 2*a4-a2=12 , again a4=2*a3 .. so the equation changes to 2*(2*a3)-a2=12 .. again a3=2*a2 so .. we have 2*(2*(2*a2)) - a2 =12 .. which is equal to 8a2-a2=12 ..

So , 7a2=12 .. therefore a2= 12/7 . but again a2 = 2*a1 that is 12/7 = 2*a1 .. and that means a1 = 6/7 .. simple ..right ?

[tutonaranjo]

gabriel.
amazing, thank you so much
apologies for posting all q's in one post, will break up.
cheers