Hi VJesus12,

We're told that the only gift certificates that a certain store sold yesterday were worth either $100 each or $10 each and that the store sold a total of 20 gift certificates. We're asked for the number of gift certificates worth $10 each that the store sold yesterday.

To start, it's worth noting that there's a pattern that you can take advantage of. Since we know that there were only 20 gift certificates sold, the total value of the certificates will change by a consistent amount as you increase the number of $100 certificates or the number of $10 certificates. For example:
0 $10s and 20 $100s = $2000
1 $10s and 19 $100 = $1910
2 $10s and 18 $100s = $1820
3 $10s and 17 $100s = $1730
Etc.

Notice that the decrease in the total value is consistently $90 less as you increase the number of $10 certificates by 1. You can use that pattern to your advantage....

1) The gift certificates sold by the store yesterday were worth a total of between $1,650 and $1,800.

IF... we have...
2 $10s and 18 $100s = $1820
3 $10s and 17 $100s = $1730
4 $10s and 16 $100s = $1640
There's ONLY ONE value that fits the information in Fact 1: (3 $10s and 17 $100s), so the answer to the question is 3.
Fact 1 is SUFFICIENT

2) Yesterday the store sold more than 15 gift certificates worth $100 each.

IF... we have...
2 $10s and 18 $100s = $1820 and the answer to the question is 2
3 $10s and 17 $100s = $1730 and the answer to the question is 3
Fact 2 is INSUFFICIENT

Final Answer: A

GMAT assassins aren't born, they're made,
Rich