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.