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# GMAT: Math 6

30-35, pg. 156. Created by Sal Khan.

## Want to join the conversation?

• is this about circle graphs
• Yes, The circle can be viewed alternatively as a pie chart. The angle created by a sector on the center, after it is by 360, gives the length of that sector as the fraction of the total perimeter.
(1 vote)
• Sal said "per second" but meant "per 11 seconds
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• 10 pairs = 2*10 = 20 Socks. , He lost 7 of them. Left only = 20-7 = 13. So greatest possible pair is 12/2= 6 pairs. ( Ans)
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• #33 isn't it just the lcm of the numbers from 1 through 7 ?
• You have forgotten your friend's phone number. You remember that the first part of the phone number is 621 and the last four digits include 1, 3, 5, and 9. How many phone numbers would you have to try to guarantee you call the right one?
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• You can either logically find out how many are possible or just use combinatorics (which would require you to know factorials).

If you were to think logically,
There would be 4 'unknown' numbers
Lets label these are position 1,2,3,4.

At position one, which of 1,3,5,9 could fit? It would be all 4, since its the first digit we're considering.
So 4.
At position two, since one of the set (1,3,5,9) have been used, there are only 3 more possibilities (4-1)
So 3.

Similarly,
Position 3 will have 2, position 4 will have 1.

So with all 4 positions, just multiply them out to get the total possible combinations, which is 4x3x2x1
or 24.

With the knowledge of combinations, this would be much more faster since we can directly estimate it as 4! (4 factorial).
(1 vote)
• who thinks this is useful?
(1 vote)
• just sayin, if you put up the speed and watch them all, you get lots of points, and badges!
(1 vote)
• I use the same method, as it is the one my math teacher recommended. It also makes sense and prevents you from doing complex division. Also, you don't have to waste time converting to percent. Thanks for posting that method for me