For a wedding reception, a banquet facility charges a flat rate of $20

Hi minwoswoh and Konstantin1983,

You both seem pretty clear on the idea that you don't have to perform every calculation to answer this question; that is more than just "intuition" though - it's an algebra shortcut that you'll be able to take advantage of a number of times on Test Day.

Here, the rule is "1 simple variable, 1 equation, I can solve it." By "simple", I mean that there are no exponents, absolute value signs, etc.

In Fact 1, we can create the equation:
$2,000 + 15Y = $2450

Here, Y represents the extra money that is charged for each additional guest (over the original 50 guests mentioned in the prompt). Since we're asked to figure out the value of Y, we CAN get the value of Y from this equal (without having to actually do the work) - "1 variable, 1 equation, I CAN solve it."
Fact 1 is SUFFICIENT

In Fact 2, we have a different formula, but a similar situation:
$2,000 + 50Y = $2,000 + 30Y + $600

Again, we have "1 variable, 1 equation", although you might need to do one extra "clean up" step to see it:

20Y = $600
Fact 2 is SUFFICIENT

Final Answer:

Be on the lookout for this pattern (and a few others like it) - they can save you some time and effort on Test Day.

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