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CAHSEE Math Videos 210 videos
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CAHSEE Math 5.1 Number Sense 190 Views
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Description:
Number Sense: Drill Set 5, Problem 1. Solve the equation.
Transcript
- 00:03
And here's your shmoop du jour.
- 00:06
1/3 plus 2/9 minus 5/12 equals... what?
- 00:10
And here are the potential answers...
- 00:16
Riddle us this: what is the least common denominator of 3, 9, and 12?
- 00:21
The easiest way to solve these LCD problems is to first frame them.
Full Transcript
- 00:30
That is, 'at worst', we can multiply the two terms...
- 00:33
...like... 9 times 12 which is 108, but there's probably a lower and easier way
- 00:38
to manage term that will keep things... sane.
- 00:41
Take the larger of the two numbers and mumble to yourself like a withered old politician
- 00:45
in the 17th hour of a filibuster.
- 00:48
He mumbles 12... does 9 go into 24 evenly? No -- keep going... 36?
- 00:58
Ahh... ding ding ding! We have a winner. So 36 should be the LCD for this fraction.
- 01:04
Oh and by the way, note that we didn't even bother to worry about the 3 there -- it goes
- 01:08
into 9 AND 12 evenly, so by default it's going to work.
- 01:12
Now we just rewrite the problem with 36 as the denominator across the board:
- 01:17
One third becomes 12/36... plus 8/36... minus 15/36.
- 01:31
And the answer is 5/36, or C.
- 01:34
Phew. Now no one needs to be carted off to the loony bin.
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