Mike drove 30

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sanju09: GMAT Instructor; Posts: 3650; Joined: Wed Jan 21, 2009 4:27 amFollowed by: 80 members; Location: India; Thanked: 267 times; GMAT Score:760

by sanju09 » Sat Jun 19, 2010 2:48 am

Mike drove 30 miles, at a constant speed, for t hours and then drove y miles, at another constant speed, for 1 hour and 15 minutes. What was his average speed, in miles per hour, for the whole journey?
(A) (30/t + y/1.25) / 2
(B) (30 + y) / (t + 1.25)
(C) (30 + y) / t
(D) (30 + y) / 1.25
(E) (30 + y) / 2(t + 1.25)

The mind is everything. What you think you become. –Lord Buddha

Sanjeev K Saxena
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Source: Beat The GMAT — Problem Solving | Sat Jun 19, 2010 2:48 am

amising6: Master | Next Rank: 500 Posts; Posts: 294; Joined: Wed May 05, 2010 4:01 am; Location: india; Thanked: 57 times

by amising6 » Sat Jun 19, 2010 2:53 am

sanju09 wrote:Mike drove 30 miles, at a constant speed, for t hours and then drove y miles, at another constant speed, for 1 hour and 15 minutes. What was his average speed, in miles per hour, for the whole journey?
(A) (30/t + y/1.25) / 2
(B) (30 + y) / (t + 1.25)
(C) (30 + y) / t
(D) (30 + y) / 1.25
(E) (30 + y) / 2(t + 1.25)

let constant speed for forst part of journey=s1=30/t
constant speeed for second part of journey s2=y/(5/4) s2=4y/5
so average speed =s1+s2/2
so (30/t+y/1.25)/2

Ideation without execution is delusion

kvcpk: Legendary Member; Posts: 1893; Joined: Sun May 30, 2010 11:48 pmFollowed by: 7 members; Thanked: 215 times

by kvcpk » Sat Jun 19, 2010 2:54 am

sanju09 wrote:Mike drove 30 miles, at a constant speed, for t hours and then drove y miles, at another constant speed, for 1 hour and 15 minutes. What was his average speed, in miles per hour, for the whole journey?
(A) (30/t + y/1.25) / 2
(B) (30 + y) / (t + 1.25)
(C) (30 + y) / t
(D) (30 + y) / 1.25
(E) (30 + y) / 2(t + 1.25)

avg speed = total dist /total time

total dist = 30 + y
total time = t + 1.25

so avg speed = 30 +y /t + 1.25

B

sanju09: GMAT Instructor; Posts: 3650; Joined: Wed Jan 21, 2009 4:27 amFollowed by: 80 members; Location: India; Thanked: 267 times; GMAT Score:760

by sanju09 » Sat Jun 19, 2010 2:57 am

amising6 wrote:
sanju09 wrote:Mike drove 30 miles, at a constant speed, for t hours and then drove y miles, at another constant speed, for 1 hour and 15 minutes. What was his average speed, in miles per hour, for the whole journey?
(A) (30/t + y/1.25) / 2
(B) (30 + y) / (t + 1.25)
(C) (30 + y) / t
(D) (30 + y) / 1.25
(E) (30 + y) / 2(t + 1.25)
let constant speed for forst part of journey=s1=30/t
constant speeed for second part of journey s2=y/(5/4) s2=4y/5
so average speed =s1+s2/2
so (30/t+y/1.25)/2

Do never confuse between arithmetic mean of speeds and the average speed.

The mind is everything. What you think you become. –Lord Buddha

Sanjeev K Saxena
Quantitative Instructor
The Princeton Review - Manya Abroad
Lucknow-226001

www.manyagroup.com

amising6: Master | Next Rank: 500 Posts; Posts: 294; Joined: Wed May 05, 2010 4:01 am; Location: india; Thanked: 57 times

by amising6 » Sat Jun 19, 2010 2:59 am

sanju09 wrote:
amising6 wrote:
sanju09 wrote:Mike drove 30 miles, at a constant speed, for t hours and then drove y miles, at another constant speed, for 1 hour and 15 minutes. What was his average speed, in miles per hour, for the whole journey?
(A) (30/t + y/1.25) / 2
(B) (30 + y) / (t + 1.25)
(C) (30 + y) / t
(D) (30 + y) / 1.25
(E) (30 + y) / 2(t + 1.25)
let constant speed for forst part of journey=s1=30/t
constant speeed for second part of journey s2=y/(5/4) s2=4y/5
so average speed =s1+s2/2
so (30/t+y/1.25)/2
Do never confuse between arithmetic mean of speeds and the average speed.

meaning average speed is total distance travelled by total time taken

so B

Ideation without execution is delusion

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