Use of basic concepts to solve questions in seconds

Q. With the speed of 50 Km/hr Ram reaches his office from home at 11:30 AM, and with speed of 90 Km/hr he reaches at 10:30 AM, what should be his speed if he wants to reach at 10:59 AM (in all cases he starts at same time and and reaches at same day)

a] 70 KM/hr
b] 70.5 Km/hr
c] 71.5 KM/hr
d] 72.5 Km/hr
e] None of these
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Sol: We know that we should use speed average, as shown in video http://www.youtube.com/watch?v=q0Y1YjwFol8 (watch this on non mobile device for text and animations and in full mode)


Now come to solution part:

we know speed average is HM. and we also know HM is less than AM (mean) 

and given speeds are 50 and 90, so AM (50,90) = 70 its AM but we are looking at HM

so, answer would be less than 70

check option less than 70 and eliminate the options, final answer is None of these
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NOTE : THESE TYPE OF QUESTIONS, FIND AM THEN ELIMINATE OPTIONS LESS THAN AM (since AM is easy to find)




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