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

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