Five friends play blackjack in Las Vegas and lose an average of $100

STRATEGY: As with all GMAT Problem Solving questions, we should immediately ask ourselves, Can I use the answer choices to my advantage?
In this case, we can easily test the answer choices, especially since we can use some number sense to eliminate 3 of the answer choices.
Now we should give ourselves up to 20 seconds to identify a faster approach.
In this case, we can also solve the question algebraically.
Both approaches seem pretty fast so let's do both and see which one we think is fastest

APPROACH #1: Test the answer choices
If two friends lost a total of $380, then their average loss is $190
So, in order for all five friends to have an average loss of $100, the average loss of the remaining three friends must be less than $100.
So, the correct answer is either A or B, which means we need only test ONE answer choice.
For example, if we test answer choice A, and it works, then we're done. If we test answer choice A, and it doesn't work, then we're still done because the answer would be B.

Let's test choice A.
If the remaining three friends had an average loss of $40, it could be the case that each of those three people lost $40.
So, the total loss of all five friends = 40 + 40 + 40 + 380 = 500, which means the average loss = 500/5 = 100. PERFECT!

Answer: A

APPROACH #2: Apply algebra
If all five friends have an average loss of $100, then we can write: (total loss for all 5 friends)/5 = 100
Multiply both sides of the equation by 5 to get: total loss for all 5 friends = $500
Since two friends lost a total of $380, we know that: total loss of the remaining 3 friends $500 - $380 = $120
So, the average loss of those remaining three friends = $120/3 = $40

Answer: A