* Arithmatic progression problem..

[mahermir]

Can anyone solve this problem?


In an aritmatic progression t1,t2,t3........tn,.......t1 = 23 and
tn=tn-1 -3 for each n>1.If tn=-4 then n=?

Ans: 10

[gokul_nair1984]

Substituting n=2 in tn=tn-1 -3 we get,

t2=t1-3

Given t1=23,

=>t2=20

Similarly,

t3=t2-3 =>t3=17

So on and so forth,

t4=14
t5=11
t6=8
t7=5
t8=2
t9=-1
t10=-4...Therefore n=10

....................
Alternatively, remember the formula for nth term of an AP

an=a1+(n-1)d

a1=23, an=-4,common difference d=-3,

You will get n=10

[RonPurewal]

is this really a problem from the GMAT PREP software? (this is the GMAT PREP folder; only problems from that software should be posted in this folder.)

i don't think the gmat expects students to know the term "arithmetic progression"; therefore, any problem containing that term is likely not to be an official problem.

therefore, if this is an official problem, please post a screenshot of the problem from the software. if it is not, kindly state the source, so that we can move it to the appropriate folder.

if you do neither of these two things, we'll have to delete the thread in a week or two. thanks.