In xy plane, what is the y intercept of line l ?
1) The slope of line l is 3 times y intercept.
2) The x intercept of line l is 1/3.

Please explain how to solve this.

Source: GMAT Prep

is the correct answer 'C' ?

eqn. for st. line is y=mx+b

fm 1) we have m=3b ; Insufficient hence we can rule out A&D

fm 2) we have 0=1/3 m +b => 1/3 m=-b ; insufficent so we can rule out B

using 1&2 we get 1/3(3b)=-b => 2b=0 or b=0; sufficient hence C

(1)
y = mx + b

substitute m = 3b --> y = 3bx + b
this won't help us find b, so, insufficient.

(2)
graphically, we can draw a line through (1/3, 0) with absolutely any slope we want. each of those lines will have a different y-intercept (with the exception of the one purely vertical line, which won't have a y-intercept at all).
insufficient.

(together)
substitute (1/3, 0) into y = 3bx + b
0 = b + b
0 = 2b
b = 0

this means the y-intercept is 0.
this is actually somewhat problematic, because we get a line with a y-intercept of 0 and a slope of 0. that line would be the x-axis; that's a problem, because statement (2), which says THE x-intercept, is now a lie (every single point on the x-axis happens to be an x-intercept of this line).

are you sure you've copied this problem correctly? or, if you have, are you sure it's from the OFFICIAL gmatprep software?

yes I'm 100% sure i have copied it correctly. Also I faced this problem while taking GMATPrep test.

From my GMATPrep software, statement 2 says the x-int is negative 1/3, not positive 1/3 (as is reported above). Can you check your GMATPrep again please?