monir6000 wrote:
In the xy plane, line a and b have the same slope. If the y intercept of line a is -1. What is the y-intercept of line b?
(1) the x-intercept of line a is -1
(2) line b passess through the point (10, 20)
given that line a and b are having the same slope... and given the line a y-intercept is -1...we need to find y-intercept of line b
1) x intercept of line a is -1
any line equation is given by y=mx+a for line a
y=mx+b for line b
hence the equation becomes y = mx - 1
for x intercept y becomes 0
0 = mx - 1
mx = 1
so for x intercept to be - 1 then to make that happen x = 1/m = -1
so m should be -1
but this makes the line b equation as y = -x + b
beyond this we cant find... hence not sufficient.. so A and D options are off...
2) line b passes through (10, 20)
y = mx + b
so 20 = 10m + b
two unknowns...we cannot solve
together.. we know that m = - 1 and 20 = 10m + b
so 20 = -10 + b
b = 30 which is the y-intercept of line b
hence C
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Regards,
Harsha
Note: Give me kudos if my approach is right , else help me understand where i am missing.. I want to bell the GMAT Cat Satyameva Jayate - Truth alone triumphs