Khan/khan-exercises — Issue #4313: Rate problems 1 - Rate problems 1 Answers 
Author: KhanBugz
Created: 2011-10-13 20:35:44+00:00
State: open
Labels: chrome, xp

Reporter: joshsmellsfunny@gmail.com

Completing this exercise is an exercise...in futility. You ask us to round the numbers and when we do, the answers frequently come out different than what you consider correct by 1. Take this problem for example:

William traveled by scooter at an average speed of 12 miles per hour.

Then, he traveled by motorcycle at an average speed of 43 miles per hour.

In total, he traveled 70 miles for 2 hours.

How many minutes did William travel by scooter? (Round to the nearest minute.)

Remember that d=r∗t, or written another way, t=d/r.

ds= distance that William traveled by scooter

dm= distance that William traveled by motorcycle

Total distance: ds+dm=70

Total time: ts+tm=2

ts=(ds/12) and tm=(dm/43)

Substitute the blue equations for: (ds/12)+(dm/43)=2
Multiply the above equation by -12: −ds−(12/43)∗dm=−24

Add the two orange equations for: (31/43)∗dm=46
Calculating all variables (and then rounding to the nearest integer):
dm= 63.8065 (64) miles
ds= 6.1935 (6) miles
ts= 30.9677 (31) minutes
tm= 89.0323 (89) minutes

ts=31 minutes

You have the answer as 31 Minutes, when if you do as you request and round the numbers, the answer comes out to 30 minutes.

ts=(ds/12)= 6/12= 1/2 * 60 minutes = 30 minutes.

When you are off, even by one, the entire exercise resets and I have to start over. It is annoying.

[rate_problems_1.html?seed=133&problem=2](http://sandcastle.khanacademy.org/media/castles/Khan:master/exercises/rate_problems_1.html?seed=133&problem=2)

["30"]

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