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gmatgg
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Bunuel
1. How many hours did it take Helen to drive from her house to her parent's house?

(1) Helen's average speed on this trip was 72km/hr
(2) If Helen's average speed on this trip had been 8km/hr greater, it would have taken her 1 hour less.

\(d=rt\), where \(d\) is the distance covered, \(r\) rate and \(t\) time. Question: \(t=?\)
(1) \(r=72\). Clearly not sufficient.
(2) \((r+8)(t-1)=d\). Clearly not sufficient.

(1)+(2) \(r=72\) and \((r+8)(t-1)=d\) --> \(80(t-1)=d\) --> \(d=80(t-1)=rt=72t\) --> \(8t=80\) --> \(t=10\). Sufficient.

Answer: C.

2. Lines n and p lie in the xy-plane. Is the slope of line n less than the slope of line p?

We have two lines: \(y_n=m_1x+b_1\) and \(y_p=m_2x+b_2\). Q: \(m_1<m_2\) true?

(1) Lines n and p intersect at the point (5,1) --> \(1=5m_1+b_1=5m_2+b_2\) --> \(5(m_1-m_2)=b_2-b_1\). Not sufficient.
(2) The y-intercept of line \(n\) is greater than the y-intercept of line \(p\) --> y-intercept is value of \(y\) for \(x=0\), so it's the value of \(b\) --> \(b_1>b_2\) or \(b_2-b_1<0\). Not sufficient.

(1)+(2) \(5(m_1-m_2)=b_2-b_1\), as from (2) \(b_2-b_1<0\) (RHS), then LHS (left hand side) also is less than zero \(5(m_1-m_2)<0\) --> \(m_1-m_2<0\) --> \(m_1<m_2\). Sufficient.

Answer: C.


Great Explanation! Thanks so much
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Bunuel
1. How many hours did it take Helen to drive from her house to her parent's house?

(1) Helen's average speed on this trip was 72km/hr
(2) If Helen's average speed on this trip had been 8km/hr greater, it would have taken her 1 hour less.

\(d=rt\), where \(d\) is the distance covered, \(r\) rate and \(t\) time. Question: \(t=?\)
(1) \(r=72\). Clearly not sufficient.
(2) \((r+8)(t-1)=d\). Clearly not sufficient.

(1)+(2) \(r=72\) and \((r+8)(t-1)=d\) --> \(80(t-1)=d\) --> \(d=80(t-1)=rt=72t\) --> \(8t=80\) --> \(t=10\). Sufficient.

Answer: C.

2. Lines n and p lie in the xy-plane. Is the slope of line n less than the slope of line p?

We have two lines: \(y_n=m_1x+b_1\) and \(y_p=m_2x+b_2\). Q: \(m_1 [m]1=5m_1+b_1=5m_2+b_2\) --> \(5(m_1-m_2)=b_2-b_1\). Not sufficient.
(2) The y-intercept of line \(n\) is greater than the y-intercept of line \(p\) --> y-intercept is value of \(y\) for \(x=0\), so it's the value of \(b\) --> \(b_1>b_2\) or \(b_2-b_1 [m]m_1-m_2 [m]m_1<m_2\). Sufficient.

Answer: C.

great explanation Bunuel, I would have never thought to compare algebraically. I solved this using a graph, and found that slope of P would be greater than N no matter if N had a higher y intercept. Even if N was negative or positive P would have a larger slope in both cases.



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