mgmt_gmat wrote:If r is a constant and an = r·n for all positive integers n, for how many values of n is an <
100?
(1) a50 = 500
(2) a100 + a105 = 2,050

Explain.

an = rn and r is a constant => a is a constant and a= r

1) a*50 = 500
a =10 ; Sufficient to answer the question for all positive integers n, for how many values of n is an <
100

2) 100a + 105a = 2050
205a = 2050

a=10 ; sufficient
[spoiler]
Hence D[/spoiler]

Ok everyone I am going to pass along some excellent advice that my more excellent tutor gave me:
SPEND MORE TIME ON REDUCING THE QUESTION STEM BEFORE STARTING THE PROBLEM!!!!!!!!

This a huge case in point.
Step 1 = Understand the question stem
If r = Constant and an=rn then a=r;
*** Once you understand that a=r now you can evaluate the question with the givens

Step 2 = Evaluate both Givens
(1) if a50=500; then a=10 - so for all 10n we can sove for how many vlues of n < 100
#1 is Sufficient

(2) If a100+a105=2050; then a=2050/205=10 - so for all 10n we can sove for how many vlues of n < 100
#2 is Sufficient

The Answer is "D"

mgmt_gmat wrote:If r is a constant and an = r·n for all positive integers n, for how many values of n is an <
100?
(1) a50 = 500
(2) a100 + a105 = 2,050

Explain.

Also see

https://www.beatthegmat.com/og-12th-edit ... 51725.html