Hi swerve,

We're told that Chris saved $400 of his earnings last month. We're asked for the total amount Chris EARNED last month.

1) Chris spent 1/3 of his last month's EARNINGS on rentals and saved 1/4 of the remainder.

Fact 1 tells us that 1/4 of 2/3 of Chris's earnings were saved. We can combine this information with what we were told in the prompt (that Chris Saved $400) and 'work backwards' to find the total earned.

$400 = 1/4 of the 'remainder'
$1600 = the 'remainder'

the 'remainder' = $1600 = 2/3 of the total earned
$1600 = (2/3)(X)
$2400 = X
Thus, Chris earned $2400, spent 1/3 of that (re: $800) on rentals and had a 'remainder of $2400-$800 = $1600. He saved 1/4 of that remainder (which matches the $400 that the prompt stated was the savings).
Fact 1 is SUFFICIENT

2) Last month, Chris paid three times as much in loan payments as he saved.

With Fact 2, we can determine the amount of money that Chris spent on loan payments (3 times $400 = $1200), but we don't know if Chris spent money on anything ELSE. Fact 2 ultimately tells us that Chris earned AT LEAST $1200+$400 = $1600, but he could have earned more than that.
Fact 2 is INSUFFICIENT

Final Answer: A

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